qemu: add support for librbd layered encryption

qemu: add support for librbd layered encryption

This commit enables libvirt users to use layered encryption
of RBD images, using the librbd encryption engine.
This allows opening of an encrypted cloned image
whose parent is encrypted with a possibly different encryption key.
To open such images, multiple encryption secrets are expected
to be defined under the encryption XML tag.

Signed-off-by: Or Ozeri <oro@il.ibm.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>