qapi: change 'unsigned special_features' to 'uint64_t features'

qapi: change 'unsigned special_features' to 'uint64_t features'

The "special_features" field / parameter holds the subset of schema
features that are for internal code use. Specifically 'DEPRECATED'
and 'UNSTABLE'.

This special casing of internal features is going to be removed, so
prepare for that by renaming to 'features'. Using a fixed size type
is also best practice for bit fields.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250205123550.2754387-3-berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>