scripts: add script to help distros use global Rust packages

Some distros prefer to avoid vendored crate sources, and instead use
local sources from e.g. ``/usr/share/cargo/registry``.  Add a
script, inspired by the Mesa spec file(*), that automatically
performs this task.  The script is meant to be invoked after unpacking
the QEMU tarball.

(*) This is the hack that Mesa uses:

    export MESON_PACKAGE_CACHE_DIR="%{cargo_registry}/"
    %define inst_crate_nameversion() %(basename %{cargo_registry}/%{1}-*)
    %define rewrite_wrap_file() sed -e "/source.*/d" -e "s/%{1}-.*/%{inst_crate_nameversion %{1}}/" -i subprojects/%{1}.wrap
    %rewrite_wrap_file proc-macro2
    ... more %rewrite_wrap_file invocations follow ...

Reviewed-by: Neal Gompa <ngompa@fedoraproject.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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