app: create a window for VTE displays
virt_viewer_app_display_added() now handles VTE displays. They should
be skipped for monitor configuration, and they don't emit "show-hint".
(a VTE display has a monitor nth == -1, which is now a valid value)
The associated window will be hidden when virt-viewer is started.
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>