I just wrote a small Bash script for creating offline-backups of a bunch of virtual machines on a server using btrfs snapshots.
The script shutsdown all running KVM machines, waits until they are down, creates a (readonly) btrfs snapshot and spins the machines back up. All together takes less than a minute. After the process I have an image of all KVM machines in the state, when the machines are shut down. This is then suitable for storing the machine image files on a different machine to have a complete working state of the machines. This is part of my backup (more hardware failure) strategy for one of our general purpose servers at work.
The KVM instances need to be in a btrfs subvolume, otherwise it doesn't work
See the script as gist on GitHub. You will need to do some adjustments and probably test it a couple of times, until it will work nicely.