Plugin should throw error when user tries to assign subport to VM

[jbemmel]

A trunk consists of a parent port and sub-ports; the user is supposed to bind the VM to the parent port.
When both a trunk port and one of its sub-ports ( device_owner: trunk:subport ) are connected to a VM, libvirt fails due to allocating the same VF / PCI ID twice.

The trunk driver should be able to detect this user error, and abort the flow with a clear error