I did that very thing about a year ago when I still had QubesOS installed, and it did not work. There seems to be a lot of misinformation about this swirling around the web. It simply does not work. There is a post somewhere that confirms it but I don't have the link. Unless the QubesOS devs/maintainers made a 180 degree turn since I tried it and decided to start compiling QubesOS with xen nested virtualization enabled, but I doubt it because their reason was that xen's nested virtualization feature is basically broken anyway.