avocado.plugin.xunit: Pass type, traceback and system output.

Pass type, traceback and system output to the result XML.
Add function to escape/quote XML attributes.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #36 from ruda/test_result_junit

avocado.plugin.xunit: Pass type, traceback and system output.