qemu snapshot function

Immutable PC

If you fear damaging your disk.img file, you can use the snapshot parameter to refrain from making modification to the diskimage. For the guest system, the disk will still appear to be writable.

qemu -snapshot -hda disk.img

Incremental diskimages

Or, you could create one or more new diskimages which are based on the previous one. Modifying these "child" disks, doesn't affect the parent disk.

qemu-img create -b disk.img -f qcow2 testdisk.img
qemu -hda testdisk.img

If you think "child" disk is ok and want to merge it into "parent" disk:

qemu-img commit -f qcow2 testdisk.img