qdict_iter() has just three uses and no test coverage. Replace by
qdict_first(), qdict_next() for more concise code and less type
punning.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200415083048.14339-5-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>