Use new XENBUS CACHE interface rather than local pool code

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Remove much code duplication

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Use new GNTTAB interface

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge pull request #32 from pauldu/gnttab

Updated GNTTAB interface