qapi: Make output visitor return qnull() instead of NULL

Before commit 1d10b44, it crashed.  Since then, it returns NULL, with
a FIXME comment.  The FIXME is valid: code that assumes QObject *
can't be null exists.  I'm not aware of a way to feed this problematic
return value to code that actually chokes on null in the current code,
but the next few commits will create one, failing "make check".

Commit 481b002 solved a very similar problem by introducing a special
null QObject.  Using this special null QObject is clearly the right
way to resolve this FIXME, so do that, and update the test
accordingly.

However, the patch isn't quite right: it messes up the reference
counting.  After about SIZE_MAX visits, the reference counter
overflows, failing the assertion in qnull_destroy_obj().  Because
that's many orders of magnitude more visits of nulls than we expect,
we take this patch despite its flaws, to get the QMP introspection
stuff in without further delay.  We'll want to fix it for real before
the release.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1442401589-24189-21-git-send-email-armbru@redhat.com>
