Shrink qcow2 file with Windows guest

1. Remove unnecessary file and empty the recycle bin in windows guest.
2. De-fragment your disk in windows guest. You may do it several times for good compact status.
3. Free unused disk space in windows guest.
   sdelete -c c:
4. Convert the disk image itself.
   qemu-img convert -c -O qcow2 win.qcow2 win-shrinked.qcow2