Re: [Libguestfs] [libnbd PATCH v2 2/5] states: Wire in a read callback

If any callback fails, and if no prior error was set, then the callback's failure becomes the failure reason for the overall read. (Note that this is different from the block status callback, which for now quits using the callback on subsequent chunks if an earlier chunk failed - we may decide to change one or the other for consistency before the API is stable.) Nothing actually passes a callback function yet, so for now this is no functional change; but this will make it possible for the next patch to add an 'nbd_aio_pread_structred' API.