I needed %gs and %crX, so I pull those from QEMU's QMP via
the 'info registers' command.  But rather than actually parsing
the JSON responses, I just look for a few specific strings to grab
the register values.  Well, I neglected to account for asynchronous
notifications, and those were causing the register reads to "fail".
So, now we just keep reading until we see what we expect.  Still
could be racy cause we read to a fixed buffer, but it's really
unlikely.  Still no time to write a JSON message parser.