sheepdog: Mark sd_snapshot_delete() lossage FIXME

sd_snapshot_delete() should delete the snapshot whose ID matches
@snapshot_id and whose name matches @name.  But that's not what it
does.  If @snapshot_id is a valid ID, it deletes the snapshot with
that ID, else it deletes the snapshot with that name.  It doesn't use
@name at all.  Add suitable FIXME comments, so someone who actually
knows Sheepdog can fix it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
