fix(vrtManager): libvirt connections open within a time limit

libvirt has no timeout for opening a connection. Measured in the test
setup: an unreachable host blocked qemu+tcp for 135 s and qemu+ssh for
133 s; a daemon that accepts and never answers blocks forever. Gunicorn
runs sync workers and the instance list connects to every compute, so
one unreachable compute held a worker for minutes per page load.

- A connection is opened in a helper thread, at most one per
  connection at a time. The request waits LIBVIRT_CONNECT_TIMEOUT
  (default 5 s). An open nobody waits for any more keeps running; until
  it ends, requests get its error at once, and if it succeeds later the
  connection is used. A failed open is not repeated for
  LIBVIRT_RETRY_AFTER (default 30 s). A stuck host holds one thread and
  does not keep other hosts from connecting.
- qemu+ssh runs ssh through vrtManager/libvirt-ssh (the URI's command
  parameter), which adds ConnectTimeout=10 and BatchMode=yes, so an
  abandoned ssh process ends and no prompt can hang. The user's ssh
  configuration (aliases, ProxyJump, keys) still applies.
- The close callback acts only for the connection it was registered
  for: a late callback of a replaced connection dropped its successor.
  The reason is reported as text. A connection that closed before it
  was published is an error.
- Compute.status is a bool (it returned the exception object, so the
  "host is down" check in refresh_instance_database never fired); the
  message is Compute.connection_error.

Unit tests cover the timeout, retry delay, late success, single open,
a stuck host next to a healthy one and the close callback. Live tests
(vrtManager/test_live_connection.py), for TCP and SSH computes of the
test setup: an unreachable address (no ssh left after 10 s), an endpoint
that never answers, a slow open completing later, a host that goes
silent on an established connection (keepalive ends calls within 45 s,
the next request reconnects), an ssh_config alias and a ProxyJump host.

vrtManager/libvirt-ssh

@@ -0,0 +1,12 @@
+#!/bin/sh
+# ssh for libvirt qemu+ssh connections (the URI's "command" parameter).
+# libvirt passes no connect timeout, so an unreachable host would keep this
+# ssh process waiting for the kernel's TCP timeout. BatchMode makes ssh fail
+# instead of waiting for a password or host key prompt nobody can answer.
+# The timeout is fixed: libvirt passes this script no environment or extra
+# arguments. It bounds only ssh's TCP connect; WebVirtCloud stops waiting
+# after LIBVIRT_CONNECT_TIMEOUT on its own, so an abandoned ssh lives at most
+# this long.
+# Everything else comes from the user's ssh configuration (aliases,
+# ProxyJump, keys).
+exec ssh -o ConnectTimeout=10 -o BatchMode=yes "$@"