[Libguestfs] Cargo edition problem with Debian 11

[Richard W.M. Jones]

> https://github.com/libguestfs/nbdkit/pull/19

Actually I suspect that the changes made in PR19 will get overwritten
since they touch files which are in git but are really controlled by
libvirt-ci.

I'm not sure how to make the changes in a way which are compatible
with libvirt-ci, maybe Eric or others can help with that.

Rich.

-- 
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