v3: - replace commit file with mirrored writes - address comments from round 2
v2: - use reference counting to be safe against device hotplug / bdrv_truncate - add comment about usage of timer
v3: - replace commit file with mirrored writes - address comments from round 2
v2: - use reference counting to be safe against device hotplug / bdrv_truncate - add comment about usage of timer