range: Eliminate direct Range member access

range: Eliminate direct Range member access

Users of struct Range mess liberally with its members, which makes
refactoring hard.  Create a set of methods, and convert all users to
call them instead of accessing members.  The methods have carefully
worded contracts, and use assertions to check them.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>