The internal function converts to byte-based before calling into
RBD code; hoist the conversion to the callers so that callers
can then be switched to byte-based themselves.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1468624988-423-8-git-send-email-eblake@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>