CA-120441: Fix "Unrecognised bus type" VDIs.

Issue: After the upgrade to CentOS 6.4, the "device/bus" path used to determine the host's CDROM details is no longer present which causes their name to appear as "Unrecognised bus type"

Fix: Use "device/susbsystem" to obtain SCSI details of the host CDROM drive. 

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>