xencat: use larger buffers at the high level

Unfortunately we have 3 buffers:
  - vchan is a shared buffer, this copies into
  - Lwt_io input/output channel buffers, these copy into
  - string buffers in the xencat proxy function

If benchmark this with stdin from 'dd if=/dev/zero bs=1M' and stdout redirected to '/dev/null' then it goes at about 90MiB/sec. Not too shabby, but not exactly awesome either. With this performance it makes vchan a decent private copy some files / control some services / log some data channel.

At least a Mirage application can use the FLOW signature and be more efficient (probably).