qemu: stage 1 can fail

[bbjubjub2494]

The minimal guest cannot boot out of the box. It's missing some kernel modules in the initrd. The minimal fix is

boot.initrd.availableKernelModules = [ "virtio_pci" "9p" "9pnet_virtio" ];

I recommend importing "${modulesPath}/profiles/qemu-guest.nix" anyway, but automatically including if guestType == "qemu" would be a better experience.