virsh: add nwfilter binding commands
    
    $ virsh nwfilter-binding-list
     Port Dev                              Filter
    ------------------------------------------------------------------
     vnet0                 clean-traffic
     vnet1                 clean-traffic
    
    $ virsh nwfilter-binding-dumpxml vnet1
    <filterbinding>
      <owner>
        <name>f25arm7</name>
        <uuid>12ac8b8c-4f23-4248-ae42-fdcd50c400fd</uuid>
      </owner>
      <portdev name='vnet1'/>
      <mac address='52:54:00:9d:81:b1'/>
      <filterref filter='clean-traffic'>
        <parameter name='MAC' value='52:54:00:9d:81:b1'/>
      </filterref>
    </filterbinding>
    
    Reviewed-by: John Ferlan <jferlan@redhat.com>
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>