avocado.utils.virt: Improve implementation.

Improve the current implementation by adding properties
to get status, names and domains. Add support functions
to start, stop, resume, suspend, shutdown, etc machines.

Clear documentation about what kind of snapshot: system checkpoint.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #83 from ruda/master

avocado.utils.virt: Improve implementation.