Comment 3 for bug 1314667

RMPrepUSB will run ON 32-bit and 64-bit Operating Systems.
That is not the same thing as QEMU will run a 64-bit Operating System.
There is a 64-bit version of QEMU but it is very buggy and won't even boot a 64-bit Vista/7 WinPE, so it is not included.
As I said, normally when booting a 64-bit OS under QEMU, you will see a Windows message saying that the system is not a 64-bit system. For some reason you are not seeing this on your system.