qapi: Eliminate QAPISchemaObjectType.check() variable members

We can use seen.values() instead if we make it an OrderedDict.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1446559499-26984-5-git-send-email-armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-9-git-send-email-eblake@redhat.com>
