Save this target port into an xml.dom.Element object.

def save_xml(self, doc, element):
        '''Save this target port into an xml.dom.Element object.'''
        super(TargetPort, self).save_xml(doc, element)
        element.setAttributeNS(XSI_NS, XSI_NS_S + 'type', 'rtsExt:target_port_ext')
        element.setAttributeNS(RTS_NS, RTS_NS_S + 'portName', self.port_name)