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.
Move test loading to module avocado.loader.

Move test loading from the TestRunner class to a proper
module avocado.loader and create a new TestLoader class to contain
the test discover and load there.

Create hook to allow the TestLoader class being replaceable and
expandable.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Add discover()

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