qapi/schema: add _check_complete flag

qapi/schema: add _check_complete flag

Instead of using the None value for the members field, use a dedicated
flag to detect recursive misconfigurations.

This is intended to assist with subsequent patches that seek to remove
the "None" value from the members field (which can never hold that value
after the final call to check()) in order to simplify the static typing
of that field; avoiding the need of assertions littered at many
callsites to eliminate the possibility of the None value.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240315152301.3621858-16-armbru@redhat.com>