Vchan_lwt_unix: implement an interface similar to Lwt_unix_net in conduit

The conduit Lwt_unix code is written using Lwt_io input/output channels.
Hopefully by implementing Lwt_io on top of vchan it will make the conduit
integration easier.

As a side-benefit it means Unix apps using vchan can use either Mirage FLOW
or Lwt_io, depending on which they prefer.

Signed-off-by: David Scott <dave.scott@citrix.com>