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/systemd/install.sh

@@ -4,7 +4,4 @@ SCRIPTPATH=$(dirname "$FILEPATH");
 cd "$SCRIPTPATH"
 cp webvirt-* /lib/systemd/system/
 cp webvirt /etc/default/
-# webvirt-socketiod (serial console) is not enabled by default: it does not
-# authenticate connections yet and refuses to start unless
-# SERIAL_CONSOLE_ENABLED = True is set in settings.py.
-echo Run to start services \"systemctl daemon-reload\; systemctl enable --now $(ls webvirt-* | grep -v socketiod | tr "\n" " ")\"
+echo Run to start services \"systemctl daemon-reload\; systemctl enable --now $(ls webvirt-* | tr "\n" " ")\"

conf/systemd/webvirt-socketiod.service

@@ -1,5 +1,5 @@
 [Unit]
-Description=Webvirt Cloud serial console (socketiod; requires SERIAL_CONSOLE_ENABLED = True)
+Description=Webvirt Cloud serial console (socketiod; refuses connections unless SERIAL_CONSOLE_ENABLED = True)
 Wants=network-online.target
 After=network-online.target