mdev: Fix daemon crash on domain shutdown after reconnect
    
    The problem resides in virHostdevUpdateActiveMediatedDevices which gets
    called during qemuProcessReconnect. The issue here is that
    virMediatedDeviceListAdd takes a pointer to the item to be added to the
    list to which VIR_APPEND_ELEMENT is used, which also clears the pointer.
    However, in this case only the local copy of the pointer got cleared,
    leaving the original pointing to valid memory. To sum it up, during
    cleanup phase, the original pointer is freed and the daemon crashes
    basically any time it would access it.
    
    Backtrace:
    0x00007ffff3ccdeba in __strcmp_sse2_unaligned
    0x00007ffff72a444a in virMediatedDeviceListFindIndex
    0x00007ffff7241446 in virHostdevReAttachMediatedDevices
    0x00007fffc60215d9 in qemuHostdevReAttachMediatedDevices
    0x00007fffc60216dc in qemuHostdevReAttachDomainDevices
    0x00007fffc6046e6f in qemuProcessStop
    0x00007fffc6091596 in processMonitorEOFEvent
    0x00007fffc6091793 in qemuProcessEventHandler
    0x00007ffff7294bf5 in virThreadPoolWorker
    0x00007ffff7294184 in virThreadHelper
    0x00007ffff3fdc3c4 in start_thread () from /lib64/libpthread.so.0
    0x00007ffff3d269cf in clone () from /lib64/libc.so.6
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1446455
    
    Signed-off-by: Erik Skultety <eskultet@redhat.com>
    Reviewed-by: Laine Stump <laine@laine.org>