Minor change to linkdelays to join with veth_interfaces (in addition
to interfaces) so that we return the MAC of the virtual interface not the physical interface. I do not much like this arrangement. We store veth interfaces in another dynamic table, so must join with both interfaces and veth_interfaces to see which iface each linkdelay corresponds to. If there is a veth entry use that, else use the normal interfaces entry. Maybe we should use the regular interfaces table, with type veth, entries added/deleted on the fly. I avoided that cause I view the interfaces table as static and pertaining to physical interfaces only, but the current arrangement does not scale well (although if it only ever has to scale to veth_interfaces, so be it).