Add a hypervisor configuration item

        :param instance: Hypervisor instance
        :param item: Item to add

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