plugins.xunit: Add class XmlResult.

The default output of XML is to standard output, use `--xunit-output` to create a file.
Improvements for using xUnit  in Jenkins (Junit).
Now the actual XML result is handled in class XmlResult.
Also minor fix (cosmetics) in tests.failtest.

Signed-off-by: Ruda Moura <rmoura@redhat.com>