Consistently check for unexpected QDict entries, and qdict_get_qdict()
success.  The latter doesn't tighten the test, it only makes it fail
more nicely.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-4-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>