qapi: Have each QAPI schema declare its returns white-list

qapi.py has a hardcoded white-list of command names that may violate
the rules on permitted return types.  Add a new pragma directive
'returns-whitelist', and use it to replace the hard-coded white-list.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1489582656-31133-6-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
