check-qdict: Simplify qdict_crumple_test_recursive()

Use qdict_get_qdict(), qdict_get_qlist() instead of qdict_get()
followed by qobject_to_qdict(), qobject_to_qlist().

While there, drop some redundant code.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-3-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
