Gnome Boxes on CentOS 7 with Windows 7 as a guest

I just installed CentOS 7 and tried to use Gnome Boxes to create some virtual machines. As long as I stick to other Linux distributions everything is fine. But I really do have trouble installing Windows 7 or 8 in a box. Boxes does not allow me to mount the VirtIO ISO and it doesn't look like Windows is aware of the virtual controller Boxes uses.

To workaround this I installed RedHat's Virtual Machine Manager. But, unfortunately, I do have the same problem here. I mounted the VirtIO drivers disk on a second IDE CDROM which worked. But, no matter what controller type I use (IDE, SATA, VirtIO), the Windows setup won't use any of the drivers on the disk. It keeps telling me, that there are no suitable drivers on the disk.

Any hints would bei much appreciated.