chardev-socket: do not blindly reset handlers when switching GMainContext

If the socket is connecting or connected, tcp_chr_update_read_handler will
be called but it should not set the NetListener's callbacks again.
Otherwise, tcp_chr_accept is invoked while the socket is in connected
state and you get an assertion failure.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

chardev/char-socket.c

@@ -632,7 +632,7 @@ static void tcp_chr_update_read_handler(Chardev *chr)
 {
     SocketChardev *s = SOCKET_CHARDEV(chr);
 
-    if (s->listener) {
+    if (s->listener && s->state == TCP_CHARDEV_STATE_DISCONNECTED) {
         /*
          * It's possible that chardev context is changed in
          * qemu_chr_be_update_read_handlers().  Reset it for QIO net