mod_storage_xmlarchive: Fix "user" iteration API Fixes use in prosody-migrator. Otherwise this particular API is not used much, or this would have been noticed before. Usually it is a different store like 'accounts' that is responsible for providing the authoritative list of users. Thanks Ge0rG for testing

<tr>
  <th>What is {x} {op} {y}?</th><td>
    <input name="captcha_challenge" type="hidden" value="{challenge}">
    <input name="captcha_reply" pattern="[0-9]+" required type="number">
  </td></tr>