Adding a VNIC type for physical functions

Adding a VNIC type for physical functions

This change adds a new VNIC type to distinguish between virtual and
physical functions in SR-IOV.

The new VNIC type 'direct-physical' deviates from the behavior of
'direct' VNICs for virtual functions. While neutron tracks the resource
as a port, it does not currently perform any management functions.
Future changes may extend the segment mapping functionality that is
currently based on agent configuration to include direct types.
However, the direct-physical VNICs will not have functional parity with
the other SR-IOV VNIC types in that quality of service and port security
functionality is not available.

APIImpact
DocImpact: Add description for new 'direct-physical' VNIC type.

Closes-Bug: #1500993

Change-Id: If1ab969c2002c649a3d51635ca2765c262e2d37f