Update following io-page ocamlfind package rename

Instead of
  io-page-unix
  io-page-xen

We now have
  io-page
  io-page.unix (containing only the extra C stub)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>

Related changes:
* [mirage/xen-disk#5]
* [mirage/mirage-block-unix#18]