Fix crash in timesync code

When ConnectVSockSync fails in watchWakeupNotifications(),
the returned net.Conn contains a nil value, but is not a nil interface
(ie checking it for `== nil` returns false).
This means we'll try to call `Close()` on the connection, which will
cause a nil pointer dereference.
See https://go.dev/doc/faq#nil_error and https://groups.google.com/g/golang-nuts/c/wnH302gBa4I
for details about this behaviour.

This bug is fixed by returning a nil interface when
`vsockDevice.Connect(uint32(port))` returns an error.

Author: cfergeau

pkg/vf/vsock.go

@@ -25,7 +25,13 @@ func ConnectVsockSync(vm *vz.VirtualMachine, port uint) (net.Conn, error) {
 	}
 	vsockDevice := socketDevices[0]
 
-	return vsockDevice.Connect(uint32(port))
+	conn, err := vsockDevice.Connect(uint32(port))
+	if err != nil {
+		// we can't `return vsockDevice.Connect()` directly, see https://go.dev/doc/faq#nil_error
+		// checking the return value for nil won't work as expected if we don't do this
+		return nil, err
+	}
+	return conn, nil
 }
 
 // connectVsock proxies connections from a host unix socket to a vsock port