chore: drop the KubeVirt guest bits

The k3s-test workers never ran (they were stuck waiting for volume
binding) and will be re-added as microVMs, so remove their KubeVirt
configs and everything that only existed for KubeVirt guests:

- hosts/k8s/k3s-test-1-worker-{1,2,3} and their outputs
- lib/genKubeVirtGuestModule.nix and lib/genK3sAgentModule.nix (the
  latter was only used by the workers)
- the KubeVirt image upload recipes (upload-vm, upload-idols,
  upload-k3s-test) and the kubevirt image package in the idols outputs
- refresh the docs; keep only accurate historical mentions (e.g. the
  restic backup path must stay as it is)

lib/genMicrovmGuestModule.nix

@@ -2,7 +2,7 @@
   pkgs,
   hostName,
   networking,
-  # vCPU count and RAM (MiB) -- mirrors the KubeVirt instancetype
+  # vCPU count and RAM (MiB) -- mirrors the former KubeVirt instancetype
   vcpu,
   mem,
   # persistent volume sizes in MiB