Fix tox coverage test result filename

We accidentally change the filename for cover/coverage.coverage.xml
Let's move it back.

Change-Id: I17baeb42ca0d06cf18ca0d9ff9c8cd970e62c40b

Author: ricolin

tox.ini

@@ -71,7 +71,7 @@ commands =
   stestr run --no-subunit-trace {posargs}
   coverage combine
   coverage html -d cover
-  coverage xml -o cover/coverage.x
+  coverage xml -o cover/coverage.xml
   coverage report --fail-under=92 --skip-covered
 
 [testenv:docs]