add serializer for vtn viewset
add serializer for vtn viewset

zdw/xos,cboling/xos,zdw/xos,cboling/xos,cboling/xos,opencord/xos,opencord/xos,open-cloud/xos,opencord/xos,cboling/xos,open-cloud/xos,cboling/xos,open-cloud/xos,zdw/xos,zdw/xos