'Retrieve the External ID(s) of any database record.
**Synopsis**: ``_get_xml_ids() -> { \'id\': [\'module.xml_id\'] }``
:return: map of ids to the list of their fully qualified External IDs
in the form ``module.key``, or an empty list when there\'s no External
ID for a record, e.g.::
{ \'id\': [\'module.ext_id\', \'module.ext_id_bis\'],
\'id2\': [] }'

@api.multi
def _get_external_ids(self):

    result = {record.id: [] for record in self}
    domain = [('model', '=', self._name), ('res_id', 'in', self.ids)]
    for data in self.env['ir.model.data'].search_read(domain, ['module', 'name', 'res_id']):
        result[data['res_id']].append(('%(module)s.%(name)s' % data))
    return result
