Then, on the QEMU host, add discard=unmap to your drive options:

Windows 7 does not natively support the high-performance VirtIO drivers used by KVM. Windows 7.qcow2 - Google Groups

To create a Windows 7 QCOW2 file, you'll need to use QEMU. Here's a step-by-step guide:

QCOW2 metadata was not flushed. Fix: Use qemu-img check to repair:

: Unlike raw disk images, QCOW2 files only occupy physical disk space as data is written, making them much smaller than the actual virtual hard drive size.