small fix for nodedev_xml

(1).__schema_name__ = "nodedev"
(2).change get_key2syspath_dict and get_key2value_dict
    from @staticmethod to method of NodedevXML.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add test for nodedev-dumpxml

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