docs: def add_conf_item(self, instance, item):
          """
          Add a hypervisor configuration item
  
          :param instance: Hypervisor instance
          :param item: Item to add
          """
          tmp_conf = {}
  
          if item not in EXTRA_CONF:
              tmp_conf['model'] = MODEL_TRANSFORM[item]
  
          for s_item in sorted(self.old_top[instance][item]):
              if self.old_top[instance][item][s_item] is not None:
                  tmp_conf[s_item] = self.old_top[instance][item][s_item]
  
          if item in EXTRA_CONF:
              tmp_conf = {item: tmp_conf}
              if len(self.topology['conf']) == 0:
                  self.topology['conf'].append(tmp_conf)
              else:
                  self.topology['conf'][self.hv_id].update(tmp_conf)
          else:
              self.topology['conf'].append(tmp_conf)
          self.hv_id = len(self.topology['conf']) - 1