vzctl: insecure ploop-based containers

It was discovered that vzctl, a set of control tools for the OpenVZ server virtualization solution, determined the storage layout of containers based on the presence of an XML file inside the container. An attacker with local root privileges in a simfs-based container could gain control over ploop-based containers. Further information on the prerequites of such an attack can be found at https://src.openvz.org/projects/OVZL/repos/vzctl/commits/...