feat(instances): a VM's RNG device and crash action in its options

Two settings the owner chose to add to Settings > Options after the
page's review with Codex. Both change the persistent definition, so
they are saved while the VM runs and used from its next start.

- Random number generator: a switch for a virtio RNG device fed from
  the host's /dev/urandom, so a new VM does not wait for entropy at
  boot. Another RNG device, or the panel's with a rate or driver, is
  shown as the VM's own and left to the XML editor.
- When the guest crashes: Restart or Stop (on_crash restart or
  destroy); libvirt's default, Stop, when the definition has none.
  Stop keeps a VM that crashes again and again down for a look. Any
  other action is shown and left to the XML editor.

instances/views.py

@@ -1749,6 +1749,34 @@ def set_guest_agent(request, pk):
     return _back(request, pk, "options")
 
 
+@require_POST
+@superuser_only
+@serialize_instance_mutation
+def set_rng(request, pk):
+    instance = get_instance(request.user, pk)
+    enabled = bool(request.POST.get("rng"))
+    try:
+        with audit_vm(request, instance, "add RNG device" if enabled else "remove RNG device"):
+            instance.proxy.set_rng(enabled)
+    except util.OperationError as err:
+        messages.error(request, err)
+    return _back(request, pk, "options")
+
+
+@require_POST
+@superuser_only
+@serialize_instance_mutation
+def set_on_crash(request, pk):
+    instance = get_instance(request.user, pk)
+    action = request.POST.get("on_crash", "")
+    try:
+        with audit_vm(request, instance, f"set crash action {action}"):
+            instance.proxy.set_on_crash(action)
+    except util.OperationError as err:
+        messages.error(request, err)
+    return _back(request, pk, "options")
+
+
 @require_POST
 @superuser_only
 @serialize_instance_mutation