nwfilter: export port binding concept in the public API

nwfilter: export port binding concept in the public API

When the daemons are split there will need to be a way for the virt
drivers and/or network driver to create and delete bindings between
network ports and network filters. This defines a set of public APIs
that are suitable for managing this facility.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>