Add some unittest for network_xml and do some fixup of netowk_xml

cover-letter:
libivrt_xml_unittest: add a unittest for ipxml in network_xml.
network_xml: fixup! fix some bug in ip accessors.

 virttest/libvirt_xml/network_xml.py |  6 ++++--
 virttest/libvirt_xml_unittest.py    | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)