'Get the names of vservers present, optionally filtered by type.'

def list_vservers(self, vserver_type='data'):

    query = ({'vserver-info': {'vserver-type': vserver_type}} if vserver_type else None)
    api_args = {'desired-attributes': {'vserver-info': {'vserver-name': None}}}
    if query:
        api_args['query'] = query
    result = self.send_iter_request('vserver-get-iter', api_args, enable_tunneling=False)
    vserver_info_list = (result.get_child_by_name('attributes-list') or netapp_api.NaElement('none'))
    return [vserver_info.get_child_content('vserver-name') for vserver_info in vserver_info_list.get_children()]
