[Libguestfs] Cargo edition problem with Debian 11

[Richard W.M. Jones]

Thanks everyone.  Just a note that nbdkit CI is passing right now.  I
disabled the rust plugin (only) for the troublesome distros.  Also I
fixed a bunch of other things that were broken.

Rich.

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