datapath: Add generic virtual port layer.

datapath: Add generic virtual port layer.

Currently the datapath directly accesses devices through their
Linux functions.  Obviously this doesn't work for virtual devices
that are not backed by an actual Linux device.  This creates a
new virtual port layer which handles all interaction with devices.

The existing support for Linux devices was then implemented on top
of this layer as two device types.  It splits out and renames dp_dev
to internal_dev.  There were several places where datapath devices
had to handled in a special manner and this cleans that up by putting
all the special casing in a single location.