bhyve: allow specifiying tty by fd

[Peter Grehan]

Hi Roman,
> Currently bhyve(8) allows to specify either stdio or device path as a
> TTY device. However, it could be useful to specify a TTY device by file
> descriptor, e.g. when bhyve is being executed in an automated way by
> other application, so a result of openpty(3) could be passed to it.
>
> Attached a poc patch for that. It allows to specify fd this way:
>
> bhyve <snip>  -s 31,lpc -l com1,fd=19 vm0

  Yes, useful idea, and will become more relevant in the capsicum world 
of passing fd's to a sandboxed bhyve.

  Thanks: I'll get this submitted.

later,

Peter.