avocado.plugins.xunit: Consider "not found" as error.

In the XML output, we can't use an exclusive attribute "not_found"
for tests that has been not found. So we're going to count
"not found" as error and remove the attribute "not_found".

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #196 from ruda/xunit_updates

avocado.plugins.xunit: Consider "not found" as error.