The drupal_write_record() function has been removed, as per https://www.drupal.org/node/2340291
It results in the following error when the settings form is saved.
Fatal error: Call to undefined function Drupal\xmlsitemap\drupal_write_record() in drupal-8.0.0-beta4/modules/xmlsitemap/src/XmlSitemapLinkStorage.php on line 127
Attached patch fixes this issue.