virt: add nodedev_xml into libvirt_xml module.

nodedev_xml for node device refer to http://libvirt.org/formatnode.html.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add unittest for nodedev_xml.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>