CA-129800: Use VDI id in journal names.

Use VDI id (origUuid) in journal file names instead of clone/leaf id.
This makes it possible to detect existence of journal files for a
certain VDI without needing to open and read the files for
additional information.

Signed-off-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #98 on xapi-project/sm