[Libguestfs] [libnbd] Slight API inconsistency

The callback (e.g. for `nbd_block_status`) now has a support for returning errors thanks to the last parameter (`int *error`), so it was switched to returning void. But that was not switched everywhere and some code expects it to return `int`. Yet another inconsistency is in the debug callback, which is supposed to return `void`, I guess, but due to the way the generator is implemented it's defined to return `int` instead. So my question is, should all callbacks just return nothing and if there is a need for some information to get back they will just use a pointer to some data (like with the `int *error`)? Or do we need different return types for callbacks and should `Callback` and `CallbackPersist` be defined as: `string * arg list * ret` ? Have a nice day, Martin