The specification wasn't clear and even contradict itself regarding
the calculation of the disk size. This have been clarified by Xen
commit 221f2748e8da ("blkif: reconcile protocol specification with
in-use implementations").
https://xenbits.xenproject.org/gitweb/?p=xen.git;a=commit;h=221f2748e8dabe8361b8cdfcffbeab9102c4c899
The value from the xenstore node "sectors" must be multiplied by 512
to have the size of the disk.
Calculation of LastBlock is now the same for both CD-ROM case and
generic case.
Signed-off-by: Anthony PERARD <anthony.perard@vates.tech>