librbd stacks on top of librados to provide access
to rbd images.

Using librbd simplifies the qemu code, and allows
qemu to use new versions of the rbd format
with few (if any) changes.
Reviewed-by: NChristian Brunner <chb@muc.de>
Signed-off-by: NJosh Durgin <josh.durgin@dreamhost.com>
Signed-off-by: NYehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>