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

Simplify commands for xUnit and JSON output plugins,
if you pass --xunit-output (or --json-output) than you don't
need to include the option --xunit (or --json).

Examples:

    avocado --xunit run sleeptest
    avocado --xunit-output /tmp/foo.xml run sleeptest
    avocado --xunit --json-output /tmp/foo.json run sleeptest.

Signed-off-by: Ruda Moura <rmoura@redhat.com>