rbd: avoid qemu_rbd_snap_list() memory leaks

When there are no snapshots qemu_rbd_snap_list() returns 0 and the
snapshot table pointer is NULL.  Don't forget to free the snaps buffer
we allocated for librbd rbd_snap_list().

When the function succeeds don't forget to free the snaps buffer after
calling rbd_snap_list_end().

Cc: qemu-stable@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
