+ * @param cfg the config set
+ * @param name the name of the config element
+ * @param desc a description for this item (used by xbt_cfg_help())
+ * @param type the type of the config element
+ * @param min the minimum number of values for this config element
+ * @param max the maximum number of values for this config element
+ * @param cb_set callback function called when a value is set
+ * @param cb_rm callback function called when a value is removed