[Libguestfs] [libnbd] More thoughts on callbacks and more

[Richard W.M. Jones]

This has an annoying subtlety around the fact that we can pass a
single user_data and multiple closures in one function call.

The LIBNBD_CALLBACK_FREE function would be called several times with
the same user_data in this case, which means the callback must do some
kind of reference counting before the user_data can be freed.

I propose that we split up Closure so it describes a single closure,
although that does mean that separate user_data must be passed with
each function pointer (which may not be a bad thing).

Rich.

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