datapath: Add support for "internal" ports similar to the local port.

datapath: Add support for "internal" ports similar to the local port.

The datapath has supported a simulated "local port" for a long time, but it
has never been possible to create additional ports with the same
characteristics.  One way to do this is using the veth driver, but this is
somewhat awkward, since there is no desire to create a pair of devices;
one suffices.

The immediate purpose for this feature is to allow an IP address to be put
on both a physical interface and a tagged VLAN attached to that interface
on Xen.