qapi: Recognize section tags and 'Features:' only after blank line

Putting a blank line before section tags and 'Features:' is good,
existing practice.  Enforce it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240216145841.2099240-12-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
9 files changed