Fix get_ip method in NetworkXML class.

Hi chris: 
    I found you have changed the param order of init in IPXML. And the get_ip method is out for the new '__init__'. But I can't find a commit for this change in your libvirt_xml_devices pull request, so I am not sure it's suitable to make a comment for this topic at pull-292 .
    So here the pull to point it out.