conf: net: Correctly switch how to format address fields

conf: net: Correctly switch how to format address fields

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