[Libguestfs] [PATCH libnbd] generator: Define new Closure type

** INCOMPLETE ** This is the generator change as discussed on the list already. The Python and OCaml bindings are not yet done. It passes all [C only] tests and valgrind. Note that nbd_add_close_callback is inconsistent with other closure types because it passes the user_data parameter after the function. (This is not caused by the current patch, it was already inconsistent). We decided that nbd_add_close_callback should be manually generated and not automatically generated because it should only be called from C, or perhaps more accurately it is only _needed_ from C (to support cleanup of the non-C bindings), but I don't think there's any reason not to automatically generate it. If we did generate it, then it would be an API break because these two parameters would get swapped around. Rich.