CA-120525: Add R/W flag to openfile interface

Files should not be opened RW by default and parents of VHD files can always be
opened RO.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #13 from simonjbeaumont/ca-120525

Add R/W flag to openfile interface
Update CHANGES for 0.6.2
Update CHANGES for 0.6.3