avocado.plugins: Fix garbage output on remote and vm plugins.

When using the remote or vm plugins, the JSON (or XML) result from
the remote server is coming with garbage at the end.
This commit fix this by filtering the result.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #299 from ruda/fix_bad_input_for_remote_vm

Fix garbage output on remote and vm plugins.