[Libguestfs] Note new Python API in nbdkit 1.17.3

[Richard W.M. Jones]

nbdkit 1.17.3 pushes this series which allows you to opt in to API
version 2 and has several improvements designed to reduce mandatory
copies between Python code and the server:

https://www.redhat.com/archives/libguestfs/2019-November/msg00209.html
(actual version: https://github.com/libguestfs/nbdkit/commit/a9b2637cf4f00fb8a25ffaf31ee83be5fe019ae2)

This does _not_ include Eric's counterproposal involving passing flags
as kwargs instead of bitmasks that he posted:

https://www.redhat.com/archives/libguestfs/2019-November/msg00248.html

Nevertheless we may make further changes to the API such as
incorporating kwargs (or may not).  So don't start coding to this new
API just now.  Remember that until something is released in one of the
stable (even numbered) branches of nbdkit it is still under
development and liable to change at any time.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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