testing: Enable virtio console for guests

This allows accessing the guests with `virsh console <name>`.

Using a serial console would also be possible but our kernel configs
have no serial drivers enabled, CONFIG_VIRTIO_CONSOLE is enabled though.
So to avoid having to recompile the kernels let's do it this way, only
requires rebuilding the guest images.

References #729.
testing: Ensure no guest is running when modifying images

Sometimes guests are not stopped properly. If images are then modified
they will be corrupted.