Once you convert the raw disk to qcow2 format, use "qemu-img" for creating a
snapshot from base image.

qemu-img create -f qcow2 -b <Base-Image>.qcow2 <Base-Image-Snapshot>.qcow2

The snapshot will have a size of around 200 KB. Further you will need to
install new Guest VM which uses snapshot as its disk image.

virt-install --virt-type=kvm --name=<New-Guest-VM> --ram 2048 --vcpus=2\
    --virt-type=kvm --hvm --network network=default --graphics vnc\
    --disk <Base-Image-Snapshot>.qcow2 --boot=hd --noautoconsole

Now you can start your new Guest VM.

And the snapshot will store only the changes in base image, thus saving a lot
of space on your host machine :)