In some situations might be useful to let qemu use an image that was prepared for a live snapshot. The advantage is that creating the snapshot file outside of the qemu process we can use the whole range of options provided by the format (eg for qcow2: encryption, cluster_size and preallocation). It is also possible to pre-set a relative path to the backing file (now it is created by default as absolute path). In the long run it can also avoid the danger of reimplementing qemu-img inside qemu (if we wanted to expose such options when a snapshot is requested).
-- Federico.
