CA-116892: 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/subsystem" to obtain SCSI details of the host CDROM drive.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Reviewed-by: Chandrika Srinivasan <chandrika.srinivasan@citrix.com>

GitHub: closes #66 on xapi-project/github