Specifying the test number manually is tedious and prone to errors (as
recently proven). Since we have all the necessary data to work out the
test number, let's do it automagically.
Add an --extension parameter to portablectl, and new DBUS methods
to attach/detach/reattach/inspect.
Allows to append separate images on top of the root directory (os-release
will be searched in there) and mount the images using an overlay-like
setup (unit files will be searched in there) using the new ExtensionImages
service option.
