libivrt_xml_unittest: add a unittest for ipxml in network_xml.

(1).modify the _from_scrach method to init a netxml with ipxml.
(2).add a test_ip_getter method to test getter of ipxml.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
network_xml: fix some bugs of ip accessors in network_xml.

(1).ip.xml accepts only filename or a string full of XML.
    So, ip_root should be passed to ipxml.xmltreefile.
(2).value is instance of IPXML, so there is no method of
    getroot() of it. it shouled be modified to
    value.xmltreefile.getroot()
(3).add a xmltreefile.write() operation at last in set and del method.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Merge pull request #343 from yangdongsheng/network_xml

Add some unittest for network_xml and do some fixup of netowk_xml.