spice: hook into port channel to create VTE terminals
    
    QEMU defines a few Spice port channel names in
    docs/spice-port-fqdn.txt that can be interacted with a terminal.
    
    Create VirtViewerDisplayVte display for all known terminal channel,
    and redirect read/write signals.
    
    Note that if VTE support is disabled, or if the VTE console isn't
    shown, spice-gtk will still process those port channels (discarding
    the read if VTE is disabled).
    
    Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
    Acked-by: Victor Toso <victortoso@redhat.com>