Add gntshr/gnttab bindings.

Signed-off-by: John Else <john.else@citrix.com>
Make sure we allocate an array of the correct size.

Signed-off-by: John Else <john.else@citrix.com>
Add some helper functions for handling Bigarrays.

Signed-off-by: John Else <john.else@citrix.com>