Comment 6 for bug 1744361

I think I found the answer. It looks like this was already documented in our code in a different place [1]:

        # NOTE(mriedem): vifs_already_plugged=True here, regardless of whether
        # or not we've migrated to another host, because we unplug VIFs locally
        # and the status change in the port might go undetected by the neutron
        # L2 agent (or neutron server) so neutron may not know that the VIF was
        # unplugged in the first place and never send an event.
        guest = self._create_domain_and_network(context, xml, instance,
                                        network_info,
                                        block_device_info=block_device_info,
                                        power_on=power_on,
                                        vifs_already_plugged=True,
                                        post_xml_callback=gen_confdrive)

So, we shouldn't be waiting for vif plug events if we've only locally unplugged vifs.

[1] https://github.com/openstack/nova/blob/d189e68/nova/virt/libvirt/driver.py#L8088-L8092