cgi: Fix CGI interface on Ubuntu 12.04

On this system unpack_from doesn't accept bytearray:

  File "//usr/share/mfscgi/mfs.cgi", line 135, in deserialize_tuple
    ret = struct.unpack_from(">" + format, bytebuffer)
TypeError: unpack_from() argument 1 must be string or read-only buffer, not bytearray

This commit fixes this using a buffer() view.

Change-Id: I2edbca473718a19273159c6377571dfbe3875934