fix(instances): a new VM and each clone get a VNC password of their own

Since the VM create became a job (6f0c7300) every new VM was defined with
no VNC password, and a clone copied its source's password (or its lack
of one) exactly: every clone of a template shared it. Computes listen
for VNC on all their addresses (dev/libvirt-bootstrap.sh), so anyone who
could reach a compute's VNC ports opened any of these consoles directly,
without the panel; the README's firewall rule was the only guard.

The create job asks for create.py's random 8-character password again,
and a clone's VNC graphics get a new random one. The console page sends
the password to a user with passwordless_console, which the user create
form now ticks by default next to change_password; a user without it is
asked for the password, as the permission intends. Existing VMs keep
their settings.

Live create, clone and VM page tests pass on a TCP compute.

instances/jobs.py

@@ -884,7 +884,7 @@ class CreateInstance(Kind):
                 nwfilter=data["nwfilter"],
                 net_model="virtio" if data["net_model"] == "default" else data["net_model"],
                 video=data["video"],
-                console_pass="",
+                console_pass="random",  # without one, anyone reaching the VNC port is in
                 mac=data["mac"],
                 qemu_ga=data["qemu_ga"],
                 add_cdrom=data["add_cdrom"],

instances/tests/test_create_job.py

@@ -48,6 +48,8 @@ class CreateJobTestCase(TransactionTestCase):
         op.refresh_from_db()
         CreateInstance().run(op, Context(op, SimpleNamespace(identity="wvc-test:1:a")))
         self.assertEqual(self.conn.create_instance.call_args.kwargs["uuid"], op.intent["uuid"])
+        # a console password of its own: without one, anyone reaching the VNC port is in
+        self.assertEqual(self.conn.create_instance.call_args.kwargs["console_pass"], "random")
         self.assertTrue(Instance.objects.filter(uuid=op.intent["uuid"], name="api-vm").exists())
 
     def test_the_api_takes_secure_boot_and_a_tpm(self):