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>
