Warning: Windows 7 is not plug-and-play aware for storage controllers. After conversion, you will likely encounter a bluescreen. To fix this, boot the Windows 7 ISO in repair mode and inject the VirtIO SCSI driver offline using dism .
A QCOW2 file only takes up space on your host machine as data is actually written to it. For instance, a 50GB virtual drive might only consume 13GB of actual host storage after a fresh Windows 7 install. Windows 7 Qcow2