openstack-nova: unintended file access

From the Red Hat advisory:

It was found that overwriting the disk inside of an instance with a malicious image, and then switching the instance to rescue mode, could potentially allow an authenticated user to access arbitrary files on the compute host depending on the file permissions and SELinux constraints of those files. Only setups that used libvirt to spawn instances and which had the use of cow images disabled ("use_cow_images = False" in nova configuration) were affected.