avocado.utils.virt: classes for using VMs.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
selftests/all/doc: Print full build output for better debug

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: install python-libvirt

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