qobject: New qdict_from_jsonf_nofail()

qobject: New qdict_from_jsonf_nofail()

Many uses of qobject_from_jsonf() convert JSON objects.  Create new
convenience function qdict_from_jsonf_nofail() that includes the
conversion to QDict.  The next few commits will put it to use.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-22-armbru@redhat.com>
(cherry picked from commit a193352ff9c7cd2cd07846118bc49921d0f53af8)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>