XENBUS #30

RangeSet fixes

- add spinlock to protect rangesets.
- add a Count value to check against empty condition in audit.
- use LONGLONG in ranges rather than ULONGLONG so that ranges can be
  invalidated by setting Start > End.

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #49 from benchalmers/xenbus30

XENBUS #30

Approved-By: Ben.Chalmers@citrix.com