[$] Private loop devices with loopfs

A loop device is a kernel abstraction that allows a file to be presented asif it were a physical block device. The typical use for a loop device is to mount afilesystem image stored in a file. Loop devices are global and shared betweenusers, which causes a number of problems for container workloads where theinstances are expected to be isolated from each other. Christian Braunerhas been working on this problem; he has posted a patchset solving it by adding a small virtual filesystem called loopfs.