Add simple test-case for new display-update qmp command.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220401143936.356460-4-vsementsov@openvz.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>