Allow the daemon to start even if there is no xen

Signed-off-by: David Scott <dave.scott@citrix.com>
Use io-page.unix and xen-gnt.unix in favour of xenctrl

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Merge pull request #13 from jonludlam/link-stubs

Link stubs