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>