Code review comment for lp:~johannes.erdfelt/nova/lp817265

The code in _destroy() already treats network_info as optional. Look at like 878 in nova/virt/xenapi/vmops.py

The calls to _destroy() that don't pass any network_info are only "templates" used for snapshots and migrations. As a result, they have no network info to tear down.

This patch is merely to fix an exception where one call to _destroy() doesn't pass anything for network_info. See the linked bug.