Avoid warning no XML exporter for event_loss

The event loss table can be exported only as CSV, this is why we have this warning in the demos:

`[2014-11-18 22:03:00,013 risk job #20 - WARNING MainProcess/10969] No "xml" exporter is available for "event_loss" outputs`

You can check that the warning has disappeared here: https://ci.openquake.org/job/zdevel_oq-engine/862/console