We have a more complete document on QOM but we should at least mention
the style requirements in the style guide.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20230424092249.58552-18-alex.bennee@linaro.org>