qemu-iotests: restrict 264 to qcow2 only

qemu-iotests: restrict 264 to qcow2 only

264 is unprepared to run with different formats, for example luks needs
handling keys, cloop doesn't support image creation, vpc creates image
larger than requested (which breaks "Backup completed: 5242880" in test
output).

The test is here to check nbd-reconnect feature and we actually don't
need it for all formats. Let's restrict it to qcow2 only.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20191025145023.6182-1-vsementsov@virtuozzo.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>