VLAN hotplug support
Besides the use case of "nested hypervisors", see issue #2345 and #5503, it's a use case for Virtual Firewall Appliances without nic hotplug support, like FreeBSD based appliances: pfSense / OPN. Instead of adding a new nic for every VLAN, which requires downtime (reboot), a VLAN could be hot-added on the trunk port without the need for downtime.
In the current network drivers for openvswitch this functionality would be pretty trivial to implement (live adjustments on VM). If this functionality is still that simple to implement if (openvswitch)networking would be moved to libvirt XML directly, see issue #5010, has yet to be determined. I'll look into that.