virtual-stb: Document why we use `os._exit`
virtual-stb: Flush std{out,err} before exiting

A typical usage pattern is:

    export $(stbt virtual-stb --daemonize --docker-image xxx)

to export the environment variables `VSTB_DOCKER_CID` etc, which are
printed to stdout by `stbt virtual-stb`. However, since with
`--daemonize` we use `_exit` instead of `exit`, this would
intermittently fail to work because `stbt` exited before anything was
printed.