Add the qemu-guest profile to the test VM

The initrd default set covers nvme, ahci, sd_mod and USB HID, but not
virtio. Without virtio_blk and virtio_pci the initrd never sees the
disk and stops at

  timed out waiting for device /dev/disk/by-partlabel/disk-main-root

profiles/qemu-guest.nix provides them. Real hardware needs no
counterpart; nvme is in the default set.

hosts/testvm/default.nix

@@ -1,9 +1,16 @@
 # ABOUTME: Host configuration for the QEMU/KVM test VM.
 # ABOUTME: Launcher script lives outside this repo at ~/VMs/moonix-vm.sh.
 
-{ pkgs, ... }:
+{ pkgs, modulesPath, ... }:
 
 {
+  # virtio_blk and virtio_pci are not in boot.initrd.availableKernelModules by
+  # default — that list covers nvme, ahci and USB, not virtio. Without them the
+  # initrd never sees the disk and stops at
+  # "timed out waiting for device /dev/disk/by-partlabel/disk-main-root".
+  # Real hardware needs no counterpart to this; nvme is covered.
+  imports = [ (modulesPath + "/profiles/qemu-guest.nix") ];
+
   # The launcher attaches the qcow2 as if=virtio, so the guest sees /dev/vda.
   # There is no by-id path for it — the disk only exists inside the VM.
   # Encryption stays on: this is where the LUKS layer gets exercised before it