avocado.plugins: Simplify commands --xunit/--json.

Simplify commands for xUnit and JSON output plugins:

* Remove options --xunit-output and --json-output.
* Use '-' to stdout or filename to options --xunit and --json.

Usage examples:

    avocado --xunit - run sleeptest  # goes to stdout
    avocado --xunit /tmp/foo.xml run sleeptest
    avocado --xunit - --json /tmp/foo.json run sleeptest.

Signed-off-by: Ruda Moura <rmoura@redhat.com>