Remove internal xen activations functions from the interface
xen: Gnttab.unmap_exn should surely throw an exception, given the suffix

For a well-behaved mirage app the unmap should never fail (because
a page is still in use) because we will manage our mappings carefully.
Merge pull request #38 from djs55/gnttab-unmapexn

Fix a minor mismatch in the Gnttab interface