libvirt_xml: Introduce a method get_section_string() in base

Sometimes we need to get the all element in xml, this
method will return a string of a element.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #1724 from yangdongsheng/get_section

libvirt_xml: Introduce a method get_section_string() in base