nixos/qemu-vm: Always add a virtio RNG device to the quest

mke2fs has this annoying property that it uses getrandom() to get random
numbers (for whatever purposes) which blocks until the kernel's secure
RNG has sufficient entropy, which it usually doesn't in the early boot
(except if your CPU supports RDRAND) where we may need to create the
root disk.

So let's give the VM a virtio RNG to avoid the boot getting stuck at
mke2fs.