protocol: Sleep for 1ms before reading log messages.

As explained in the comment:

  /* QEMU's console emulates a 16550A serial port.  The real 16550A
   * device has a small FIFO buffer (16 bytes) which means here we see
   * lots of small reads of 1-16 bytes in length, usually single
   * bytes.  Sleeping here for a very brief period groups reads
   * together (so we usually get a few lines of output at once) and
   * improves overall throughput, as well as making the event
   * interface a bit more sane for callers.  With a virtio-serial
   * based console (not yet implemented) we may be able to remove
   * this.  XXX
   */