WIP: Virt test (mergeable version)

Prepare for a long, hard battle to get this upstream.

Using the experience obtained in the first 2 proof of concepts, start a cleaner, better version of virt-test core libraries that will be used by the new projects `avocado-virt-plugin` and `avocado-virt-tests`. Right now (27/08), the libs are far from done, but we're progressing nicely.