[Libguestfs] [PATCH libnbd v2 0/4] api: Implement concurrent writer.

v1: https://www.redhat.com/archives/libguestfs/2019-June/msg00014.html I pushed a few bits which are uncontroversial. The main changes since v1 are: An extra patch removes the want_to_send / check for nbd_aio_is_ready in examples/threaded-reads-and-writes.c. This logic was wrong since commit 6af72b87 as was pointed out by Eric in his review. Comments and structure of examples/concurrent-writes.c has been updated to match. Callbacks now return int instead of void. In some cases we ignore this return value. I added a lot more commentary in the commit message for the main patch (now patch 3 in this series). Rich.