Comment 9 for bug 1878024

> need to consider the case where the _base is not on the same filesystem as the instances

>>> import os
>>> os.stat(CONF.instances_path).st_dev == os.stat(os.path.join(CONF.instances_path, CONF.image_cache.subdirectory_name)).st_dev

Note that os.stat() does not work for Windows as the st_dev always 0.