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: Nyangdongsheng <yangds.fnst@cn.fujitsu.com>