tests/lcitool: Generate distribution packages list in JSON format

tests/lcitool: Generate distribution packages list in JSON format

Add the generate_pkglist() helper to generate a list of packages
required by a distribution to build QEMU.

Since we can not add a "THIS FILE WAS AUTO-GENERATED" comment in
JSON, create the files under tests/vm/generated/ sub-directory;
add a README mentioning the files are generated.

Suggested-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20230711144922.67491-2-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>