I think we do not need a new module for iface. Lots of the functions are getting or setting info in xml. I believe they should be 
virttest/libvirt_xml and the others need be added into another module such as utils_net, virsh or utils_misc. I am not clear about
every function in your patch.

In addition, I sugguest you to read some modules in virttest/libvirt_xml. There are some specific coding style you need to follow.

Hmmm, it seems there are lots of work we need to do before apply this large patchset. My suggestion is splite this patch into 
some small pull-requests. We can discuss the all problem in first patchset and then the others will be easy. 

e.g:
we can focus on virsh_iface_define_undefine test. Add the apis this test will use in the correct module. Then I believe the other
tests is very similar work with it.