repart: Prefer using loop devices to populate filesystems when available

repart: Prefer using loop devices to populate filesystems when available

Let's make sure we use loop devices if we have access to them and
only fall back to regular files if we can't use loop devices. We
prefer loop devices because when using mkfs --root options, we have
to populate a temporary staging tree which means we're copying every
file twice instead of once when using loop devices.