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>
Rrd_reader: cache a single gnttab interface instead of using a new one for unmapping

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