Switch to new GNTTAB interface.

This patch will break indirect but no shipping backend actually uses that
yet.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge pull request #15 from pauldu/gnttab

Switch to new GNTTAB interface.
Fix a few build warnings

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge pull request #16 from pauldu/gnttab

Fix a few build warnings