iotests: 205: support luks format

Support default luks options in VM.add_drive and in new library
function qemu_img_create. Use it in 205 iotests.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180206182507.21753-1-vsementsov@virtuozzo.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>