Update to new merged Io_page

The opam package 'io-page' will install two ocamlfind packages:

  io-page
  io-page.unix

A Unix app needs both, a xen app only needs io-page.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #195 from djs55/new-merged-io-page

Update to new merged Io_page