qapi/ui: add 'display-update' command for changing listen address

Add possibility to change addresses where VNC server listens for new
connections. Prior to 6.0 this functionality was available through
'change' qmp command which was deleted.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220401143936.356460-3-vsementsov@openvz.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>