feat(console): a working serial console

The xterm console never connected: socketiod expected a different token
format, its worker had no compute credentials, one global console was
shared by every client and its output broadcast to all of them, and the
Docker image did not run socketiod at all.

- console/tokens.py: one authorize() for noVNC and socketiod; the signed
  token is the only credential, checked at connect time.
- socketiod: one pty worker per client (consolecallback with the
  instance id), one session per VM, a session cap, input and resize
  checked; the parent never opens libvirt. The engine.io same-origin
  check is on.
- consolecallback: opens the console with the compute's credentials and
  exits with a message when it cannot.
- Docker runs socketiod as a runit service; nginx proxies /socket.io/
  with Host and the scheme for the origin check. Supervisor and systemd
  start it as well; WEBVIRTCLOUD_SERIAL_CONSOLE=1 enables it.
- Tests: token and session unit tests, socketiod as a process, and a
  live test that reads SeaBIOS from a wvc-test VM on the TCP and SSH
  hosts.

conf/supervisor/webvirtcloud.conf

@@ -14,14 +14,13 @@ autostart=true
 autorestart=true
 redirect_stderr=true
 
-# Serial console daemon. Disabled by default because it does not authenticate
-# connections yet; set SERIAL_CONSOLE_ENABLED = True in settings.py and
-# autostart=true here only on a trusted network.
+# Serial console daemon; it refuses connections while SERIAL_CONSOLE_ENABLED
+# is off in settings.py.
 [program:socketiod]
 command=/srv/webvirtcloud/venv/bin/python3 /srv/webvirtcloud/console/socketiod
 directory=/srv/webvirtcloud
 user=www-data
-autostart=false
+autostart=true
 autorestart=true
 redirect_stderr=true