xencat: use Lwt_io.{read_into,write_from_exactly} instead of char-based I/O
xencat: use Lwt_io.{read_into,write_from_exactly} instead of char-based I/O

This should reduce the number of Lwt threads created, from 2 per byte
to 2 per block.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

mirage/ocaml-vchan,samoht/ocaml-vchan,samoht/ocaml-vchan,djs55/ocaml-vchan,mirage/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan