rbd: use the higher level librbd instead of just librados

rbd: use the higher level librbd instead of just librados

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: Christian Brunner <chb@muc.de>
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>