[Libguestfs] [PATCH libnbd] ublk: Add new nbdublk program

[Richard W.M. Jones]

FYI here is the asynch version:

https://gitlab.com/rwmjones/libnbd/-/commits/nbdublk/

I kept it as two commits so you can see the differences but obviously
it would be folded into one commit upstream.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit