Modify for net update test

Add function to get sub section of Network xml
It is used by https://github.com/autotest/tp-libvirt/pull/113
We should not use xmldoc in this module. Reason is that this module is in high-level and xmldoc is not available for it. Architecture looks like below. 
```
    --------->xmlbase----
    |                   |
    |                   |
network_xml-->accessor----->xml_utils-->element_tree--->re/string
```
The level of xmldoc is same with re/string. So network_xml is not allowed to use it. And what's more, we does not use xmldoc to parse xml file. Detail you can see element_tree.py.