block/rbd: Add support for rbd image encryption

Starting from ceph Pacific, RBD has built-in support for image-level encryption.
Currently supported formats are LUKS version 1 and 2.

There are 2 new relevant librbd APIs for controlling encryption, both expect an
open image context:

rbd_encryption_format: formats an image (i.e. writes the LUKS header)
rbd_encryption_load: loads encryptor/decryptor to the image IO stack

This commit extends the qemu rbd driver API to support the above.

Signed-off-by: Or Ozeri <oro@il.ibm.com>
Message-Id: <20210627114635.39326-1-oro@il.ibm.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>