librbd/crypto/LoadRequest: clone format for migration source image

librbd/crypto/LoadRequest: clone format for migration source image

Migration source and migration target images naturally have the same
encryption format, but the user shouldn't have to need to specify it
for the image that they can't even immediately see -- migration source
image gets moved to the RBD trash to avoid mistaken usage while
migration is in progress.

Fixes: https://tracker.ceph.com/issues/63184
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>