Tidy up the Rrd_reader interface

Signed-off-by: John Else <john.else@citrix.com>
Tidy up the Rrd_writer interface

Signed-off-by: John Else <john.else@citrix.com>
Cache a single interface to gnttab

Unmapping pages with a different interface to the one used to map them
in the first place doesn't work.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #8 from johnelse/tidy-mli

Tidy up module interfaces
Merge pull request #9 from johnelse/cache-gnttab-interface

Cache a single interface to gnttab