qdict: Make qdict_get_qlist() safe like qdict_get_qdict()

Commit 89cad9f changed qdict_get_qdict() to return NULL instead of
crash when the key doesn't exist or its value isn't a QDict.
Commit 2d6421a neglected to do the same for qdict_get_qlist().
Correct that, and update the function comments.

qdict_get_obj() is now unused, remove.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-2-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
