rbd: fix leak in qemu_rbd_open failure paths

Fix leak of s->snap in failure path.  Simplify error paths for the whole
function.

Reported-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
