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>

This needs to be merged at the same time as:
1. [mirage/mirage-block-xen#15]
2. [mirage/io-page#5]
3. (TBD) a new opam page definition for 'io-page'