virt_viewer_app_quit: Cleanly close the connection before quiting
    
    Even though the previous patches in this series ensure that the session
    gets properly finalized, we still need to wait for the disconnect signal,
    as spice-glib uses co-routines which need some time to cleanly close the
    connection / session.
    
    Signed-off-by: Hans de Goede <hdegoede@redhat.com>