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 #194 from avsm/restore-4.00.1-compat

Restore 4.00.1 compat
Merge pull request #195 from djs55/new-merged-io-page

Update to new merged Io_page
Merge branch 'master' of github.com:mirage/mirage