Fix virt-viewer --reconnect crash with SPICE VMs
    
    When using virt-viewer --reconnect, virt-viewer currently crashes when
    a SPICE VM is destroyed with "virsh destroy"
    
    What happens is that when the guest is destroyed, virt-viewer receives a
    SPICE_CHANNEL_ERROR_IO notification in
    virt_viewer_session_spice_main_channel_event().  This triggers the
    emission of the "session-disconnected" signal, which will end up calling
    spice_session_disconnect() (indirectly through
    virt_viewer_app_disconnected/virt_viewer_app_deactivate).
    
    Since spice-gtk commit ff25f3e, the actual session disconnection is
    done from an idle.  When the "session-disconnected" emission stops, the
    VirtViewerSession instance is destroyed. However, the associated
    VirtViewerDisplaySpice are still alive as the various SpiceChannels
    instances hold a reference through the "virt-viewer-displays" GObject
    data.
    These channels are destroyed when the idle queued by spice_session_disconnect()
    run. The associated VirtViewerDisplay are in turn destroyed too, but
    this causes attempts to use the VirtViewerSession associated with the
    displays, which has already been destroyed. This causes a crash.
    
    This commit adds a virt_viewer_session_spice_clear_displays() which is
    similar to virt_viewer_session_clear_displays(), but makes sure the
    "virt-viewer-displays" references are dropped too. This ensures the
    VirtViewerDisplay instances don't outlive the VirtViewerSession
    they are associated with.
    
    Backtrace for the crash:
    
     #0  0x0000000000413f0f in display_show_hint (display=0x85ab50 [VirtViewerDisplaySpice], pspec=0x939bd0 [GParamFlags], user_data=0x0) at virt-viewer-app.c:949
     #4  0x00000031ff22a29f in <emit signal notify:show-hint on instance 0x85ab50 [VirtViewerDisplaySpice]> (instance=instance@entry=0x85ab50, signal_id=<optimized out>, detail=<optimized out>) at gsignal.c:3361
         #1  0x00000031ff20fc45 in g_closure_invoke (closure=0xa98f50, return_value=return_value@entry=0x0, n_param_values=2, param_values=param_values@entry=0x7fffffffc700, invocation_hint=invocation_hint@entry=0x7fffffffc680) at gclosure.c:768
         #2  0x00000031ff2214c9 in signal_emit_unlocked_R (node=node@entry=0x674f80, detail=detail@entry=1678, instance=instance@entry=0x85ab50, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7fffffffc700) at gsignal.c:3549
         #3  0x00000031ff229ed0 in g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7fffffffc8d0) at gsignal.c:3305
     #5  0x00000031ff214175 in g_object_dispatch_properties_changed (object=0x85ab50 [VirtViewerDisplaySpice], n_pspecs=<optimized out>, pspecs=<optimized out>) at gobject.c:1056
     #6  0x00000031ff216661 in g_object_notify (pspec=0x939bd0 [GParamFlags], object=0x85ab50 [VirtViewerDisplaySpice]) at gobject.c:1149
     #7  0x00000031ff216661 in g_object_notify (object=0x85ab50 [VirtViewerDisplaySpice], property_name=<optimized out>) at gobject.c:1197
     #8  0x000000000041e5ab in virt_viewer_display_set_show_hint (self=0x85ab50 [VirtViewerDisplaySpice], mask=1, enable=0) at virt-viewer-display.c:691
     #9  0x000000000042b62d in update_display_ready (self=0x85ab50 [VirtViewerDisplaySpice])
         at virt-viewer-display-spice.c:145
     #13 0x00000031ff22a29f in <emit signal notify:ready on instance 0x898590 [SpiceDisplay]> (instance=instance@entry=0x898590, signal_id=<optimized out>, detail=<optimized out>) at gsignal.c:3361
         #10 0x00000031ff20fc45 in g_closure_invoke (closure=0x99b280, return_value=return_value@entry=0x0, n_param_values=2, param_values=param_values@entry=0x7fffffffcc50, invocation_hint=invocation_hint@entry=0x7fffffffcbd0) at gclosure.c:768
         #11 0x00000031ff2214c9 in signal_emit_unlocked_R (node=node@entry=0x674f80, detail=detail@entry=1696, instance=instance@entry=0x898590, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7fffffffcc50) at gsignal.c:3549
         #12 0x00000031ff229ed0 in g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7fffffffce20) at gsignal.c:3305
     #14 0x00000031ff214175 in g_object_dispatch_properties_changed (object=0x898590 [SpiceDisplay], n_pspecs=<optimized out>, pspecs=<optimized out>) at gobject.c:1056
     #15 0x00000031ff216661 in g_object_notify (pspec=0xa83370 [GParamBoolean], object=0x898590 [SpiceDisplay]) at gobject.c:1149
     #16 0x00000031ff216661 in g_object_notify (object=0x898590 [SpiceDisplay], property_name=<optimized out>) at gobject.c:1197
     #17 0x00007ffff7522525 in update_ready (display=0x898590 [SpiceDisplay]) at spice-widget.c:236
     #18 0x00007ffff752257e in set_monitor_ready (self=0x898590 [SpiceDisplay], ready=0)
         at spice-widget.c:244
     #19 0x00007ffff75274e6 in primary_destroy (channel=0x89f5c0 [SpiceDisplayChannel], data=0x898590)
         at spice-widget.c:2169
     #20 0x00007ffff7528918 in channel_destroy (s=0x909fa0 [SpiceSession], channel=0x89f5c0 [SpiceDisplayChannel], data=0x898590) at spice-widget.c:2484
     #24 0x00000031ff22a29f in <emit signal ??? on instance 0x909fa0 [SpiceSession]> (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>) at gsignal.c:3361
         #21 0x00000031ff20fc45 in g_closure_invoke (closure=0xa9bda0, return_value=return_value@entry=0x0, n_param_values=2, param_values=param_values@entry=0x7fffffffd280, invocation_hint=invocation_hint@entry=0x7fffffffd200) at gclosure.c:768
         #22 0x00000031ff2214c9 in signal_emit_unlocked_R (node=node@entry=0x9c17d0, detail=detail@entry=0, instance=instance@entry=0x909fa0, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7fffffffd280) at gsignal.c:3549
         #23 0x00000031ff229ed0 in g_signal_emit_valist (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7fffffffd450) at gsignal.c:3305
     #25 0x00007ffff71c3248 in spice_session_channel_destroy (session=0x909fa0 [SpiceSession], channel=0x89f5c0 [SpiceDisplayChannel]) at spice-session.c:2217
     #26 0x00007ffff71bd8b2 in session_disconnect (self=0x909fa0 [SpiceSession], keep_main=0)
         at spice-session.c:281
     #27 0x00007ffff71c1b27 in session_disconnect_idle (self=0x909fa0 [SpiceSession]) at spice-session.c:1853
     #28 0x00000031fee4a0ba in g_main_context_dispatch (context=0x6a4400) at gmain.c:3122
     #29 0x00000031fee4a0ba in g_main_context_dispatch (context=context@entry=0x6a4400) at gmain.c:3737
     #30 0x00000031fee4a450 in g_main_context_iterate (context=0x6a4400, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3808
     #31 0x00000031fee4a772 in g_main_loop_run (loop=0x9890f0) at gmain.c:4002
     #32 0x0000003babc05f75 in gtk_main () at gtkmain.c:1219
     #33 0x000000000043143b in main (argc=1, argv=0x7fffffffda48) at virt-viewer-main.c:12