virt-libvirt_vm: add a method get_key2value_from_key2path in libvirt_vm.

def get_key2value_from_key2path(key2path_dict):
    """
    Get the values stored in files whose path is in key2path_dict.

    @param key2path_dict: dict stored key and the path.
    @return: dict stored key and the value.
    """

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

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

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