Enabling remove_unused_base_images Removes Backing Images of Active VMs

Issue

• 'remove_unused_base_images' option does not respond as expected
• If 'remove_unused_base_images' is set to 'True' and the 'remove_unused_resized_minimum_age_seconds' or 'remove_unused_original_minimum_age_seconds' intervals are reached the backing file of active VMs are removed from /var/lib/nova/instance/_base/'uuid'. Removing the backing file on a running instance immediately puts the VM filesystem into read-only state and we have observed that it does not re-fetch the file when it is restarted as might be expected.