When formatting the forward mode addresses or interfaces the switch was
done based on the type of the network rather than of the type of the
individual <interface>/<address> element. In case a user would specify
an incorrect network type ("passhtrough") with <address> elements,
libvirtd would crash as it would attempt to format an <interface>.
Use the type of the individual element to format the XML.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1132347
