window: keep display size when leaving fullscreen for first time
    
    If the application was started in fullscreen, window geometry has not
    been saved, since the window was not realized. We can unfullscreen and
    restore 1:1 window to match guest display size with
    virt_viewer_display_queue_resize()
    
    https://bugzilla.redhat.com/show_bug.cgi?id=916810