Stop running coverage xml

We do not use the xml output, so it is wasted effort.

Change-Id: I03e3eded48d30a9cf8aea0ead9559417064aa272
Reviewed-on: https://review.openstack.org/19563
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins