rbd: Add support for bdrv_invalidate_cache

This fixes Ceph issue 2467: ttp://tracker.ceph.com/issues/2467

[Dropped return r in void function as suggested by Josh Durgin
<josh.durgin@inktank.com>.
--Stefan]

Signed-off-by: Adam Crume <adamcrume@gmail.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1412880272-3154-1-git-send-email-adamcrume@gmail.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>