conf: add support for parsing/formatting virNWFilterBindingDefPtr

conf: add support for parsing/formatting virNWFilterBindingDefPtr

A typical XML representation of the virNWFilterBindingDefPtr struct
looks like this:

  <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>