Qemu Snapshots
To create a snapshot of BACKING_IMG.qed run the following command. Then use SNAP_IMG.qed as the source image in the vm. All changes will be written to SNAP_IMG.qed and BACKING_IMG.qed will be unmodified.
Running the same command will create a new snapshot and discard all changes to BACKING_IMG.qed
qcow2 and qed support snapshots
qemu-img create -f qed -b BACKING_IMG.qed SNAP_IMG.qed
qemu-img create -f qed -b SNAP_IMG.qed SNAP_OF_SNAP_IMG.qed