datapath: Add 'rte_port_veth'

Add an implementation of the vEth port, a.k.a. Host KNI, for the
packet framework.

This implementation is heavily based around the implementation of
US-vHost included with Intel DPDK vSwitch.

Change-Id: I3838e38283169e0898e648797c25e24f84c42b2e
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Reviewed-by: Mark D. Gray <mark.d.gray@intel.com>
Reviewed-by: Meghan Halton <meghan.halton@intel.com>
Reviewed-by: Ian Stokes <ian.stokes@intel.com>