Adds the port type definitions and methods that will be used to bind
interfaces to the Midonet virtual ports.

virtnetdevmidonet.c adds the way to bind and unbind the ports by
calling into the Midonet Host Agent control command line (installed
with the midolman package).
Signed-off-by: NAntoni Segura Puimedon <toni+libvirt@midokura.com>