qobject: New qobject_from_vjsonf_nofail(), qdict_from_vjsonf_nofail()

qobject: New qobject_from_vjsonf_nofail(), qdict_from_vjsonf_nofail()

Every printf()-like function sooner or later needs its vprintf()-like
buddy.  The next commit will need qobject_from_jsonf_nofail()'s buddy,
and qdict_from_jsonf_nofail()'s buddy will be used later in this
series.  Add both.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20180806065344[email protected]>