nwfilter: wire up new APIs for creating and deleting nwfilter bindings
    
    This allows the virsh commands nwfilter-binding-create and
    nwfilter-binding-delete to be used.
    
    Note using these commands lets you delete filters that were
    previously created automatically by the virt drivers, or add
    filters for VM nics that were not there before. Generally it
    is expected these new APIs will only be used by virt drivers.
    It is the admin's responsibility to not shoot themselves in
    the foot.
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>