runtime test: Ensure all containers are unmounted at nuke()

Otherwise we may leave around e.g. devmapper mounts
image: Unmount before removing device in error paths

The device remove fails unless we unmount first
Add DeviceSet.HasActivatedDevice()

This lets you see if a device has been activated
Image.Changes: Deactivate image device after unmounting it

There is no need to keep the image device around if we were the
onces creating the device.
Tests: Clean up any old devmapper leftovers before starting tests