qlist_iter() has just three uses outside tests/.  Replace by
QLIST_FOREACH_ENTRY() for more concise code and less type punning.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200415083048.14339-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>