Use minidom factory methods in xml coverage generator

The exportToXML was creating an minidom.Element using
it's constructor, which is forbidden in the documentation:
> Applications should not instantiate the classes themselves; they should use the creator functions available on the Document object.
http://docs.python.org/library/xml.dom.minidom.html

Since python 3.4 this method no longer works (private interface change)
thus the xml coverage generation was broken.

Fix by calling the recommended method Document.createElement.

See bug: http://bugs.python.org/issue15290



Signed-off-by: Kevin Rocard <kevin.rocard@intel.com>