qapi: Avoid use of 'data' member of QAPI unions

qapi: Avoid use of 'data' member of QAPI unions

QAPI code generators currently create a 'void *data' member as
part of the anonymous union embedded in the C struct corresponding
to a QAPI union.  However, directly assigning to this member of
the union feels a bit fishy, when we can assign to another member
of the struct instead.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1457021813-10704-9-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>