snapshot: fix referencing wrong variable in while loop in do_delvm

The while loop variabal is "bs1",
but "bs" is always passed to bdrv_snapshot_delete_by_id_or_name.
Broken in commit a89d89d3, v1.7.0.

Signed-off-by: Zhang Haoyu <zhanghy@sangfor.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>