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/tests/test_live_xml_effects.py

@@ -125,6 +125,16 @@ EDITS = {
         lambda t: first(t, "devices/video/model/@type") == "virtio",
         ["devices/video"], True, False,
     ),
+    "set_rng": (
+        lambda vm: vm.set_rng(False),
+        lambda t: not t.xpath("devices/rng"),
+        ["devices/rng"], True, False,
+    ),
+    "set_on_crash": (
+        lambda vm: vm.set_on_crash("restart"),
+        lambda t: t.findtext("on_crash") == "restart",
+        ["on_crash"], True, False,
+    ),
     "resize_cpu": (
         lambda vm: vm.resize_cpu("1", "2"),
         lambda t: t.findtext("vcpu") == "2" and first(t, "vcpu/@current") == "1",