virsh: add nwfilter binding commands

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>