blockdev: Fail blockdev-add with encrypted images Encrypted images need a password before they can be used, and we don't want blockdev-add to create BDSes that aren't fully initialised. So for now simply forbid encrypted images; we can come back to it later if we need the functionality. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>