qemu-img commit virtioa.qcow2

cd /opt/unetlab/addons/win-7test

and perform the sparsify command:

virt-sparsify  --compress virtioa.qcow2 compressedvirtioa.qcow2

This will take some time and another compressed image will be created in the same image directory (win-7test) now you can rename your original virtioa.qcow2 file to orig.qcow2

mv virtioa.qcow2 orig.qcow2

Rename the compressed image name to virtioa.qcow2:

mv compressedvirtioa.qcow2 virtioa.qcow2

now you can test your new compressed image on a lab, just wipe the node and start it. If the compressed node works fine, you can delete your original source image: