avocado.plugins.jsonresult: JSON test result.

JSON test result output plugin, obsoletes avocado.plugins.collector.
Use parameters `--json` and (optionally) `--json-output <filename>`.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #70 from ruda/avocado_plugins_json

avocado.plugins.jsonresult: JSON test result.
avocado.utils.virt: small fix.

Signed-off-by: Ruda Moura <rmoura@redhat.com>