[Libguestfs] [PATCH libnbd v2 00/10] Callbacks and OCaml and Python persistent buffers.

This is a combination of these two earlier series: https://www.redhat.com/archives/libguestfs/2019-August/msg00235.html https://www.redhat.com/archives/libguestfs/2019-August/msg00240.html plus changes to allow .callback = NULL / .free != NULL, and to reduce the complexity of freeing callbacks. Although it's rather long there's nothing complex here. We might consider squashing some patches together. I believe I've addressed everything raised in the earlier reviews, with the exception that I'm not using Py_XDECREF because it seems a bit obscure (I'd never heard of it myself and it hides the true meaning of the statement). Rich.