Some drivers, like the Open vSwitch one, may restrict the creation of a
trunk from a bound port (i.e. prevents from creating trunks after a VM
is already booted on the port).
This limitation frees us from worrying about dataplane migrations for OVS
as trunk ports and regular ports are wired differently.
However, other drivers, like the Linux Bridge one, may not be affected by
this limitation, and the existing rule should be relaxed to allow the
operation based on whether not the driver can support it.
Partially-implements: blueprint vlan-aware-vms
Change-Id: Ia938eb7cc845f6f144924acdb0e333d23516fcca
42f2ba88a5