Even if a XenServer has had all its VMs migrated to other hosts, shutting it down with HA enabled seems to destroy all NICs and networking. Even an emergency network reset has no effect. No NICs at all show up in xsconsole.
There should be either a warning about this or that HA is running should be indicated more prominently somehow on XenCenter. Above all, a simple shutdown of a host should not cause all networking to be destroyed.