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

[Richard W.M. Jones]

** 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.