qapi: Generate in source order

qapi: Generate in source order

The generators' conversion to visitors (merge commit 9e72681d16)
changed the processing order of entities from source order to
alphabetical order.  The next commit needs source order, so change it
back.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180211093607.27351-17-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>