[Libguestfs] [PATCH libnbd v3 0/2] lib: Implement closure lifetimes.

I think I've addressed everything that was raised in review. Some of the highlights: - Callbacks should be freed reliably along all exit paths. - There's a simple test of closure lifetimes. - I've tried to use VALID|FREE in all the places where I'm confident that it's safe and correct to do. There may be more places. Note this is an optimization and shouldn't affect correctness. - It's a bit difficult to use VALID|FREE when freeing the debug callback (because context is not easily available) so I didn't make that change. Rich.