QCOW2 virtual size lowering

If you want to shrink the virtual size, you need to use virt-resize

1. You need to convert the qcow2 image to raw
   qemu-img convert -O raw guest.img guest.raw
2. Then resize the raw file
   qemu-img resize guest.raw 3G
3. Then convert it back to qcow2
   qemu-img convert -O qcow2 -o compat=0.10 guest.raw guest.img
4. Then run the guest and resize your file system