Setting up a Windows VMs with VirtIO drivers is most easily done by creating an entirely new VM, attempting to switch existing VMs using SATA or IDE will fail to boot, even if the drivers are installed ahead of time.

Both the Windows ISO and VirtIO ISO need to be mounted and all available devices set to virtio. Once in the Windows installer it will ask for an IO driver, go to the VirtIO disc and navigate to D:\virtio-win.0.1.xxx\amd64\w10 so that it can load the necessary driver. After install, run the .exe installer from the disc to install the rest of the drivers.

Convert qcow2 to vdi

qemu-img convert -O vdi <source>.qcow2 vdi_<destination>.vdi

Change the size of a qcow2 image

qemu-img resize <image>.qcow2