avocado.plugins.vm: Display Job ID properly in the CLI

Set a Job ID in the host, and force the avocado command
run using it inside the VM. With this, we get to display
that information to the user.

Also, right now, as we parse and display results all at once,
displaying the job log doesn't matter much. Therefore,
let's keep the output symmetrical to the one obtained
running avocado in the host.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.vm: Increase CLI output consistence

Reword and align messages that inform libvirt domain
and VM login.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Add Virtual Machine Plugin documentation

It was something that we forgot to do back when the feature
was implemented. This basic guide should be enough to
understand and use the feature.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>