CA-128011: Device path is sysfs has changes from Centos 5 to 6

sysfs entry for device is different in clearwater from the latest XS
versions, formulate path accorndingly so that the same xencert kit
runs on all versions from clearwater onwards

Signed-off-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>
Reviewed-by: Zheng Li <zheng.li@eu.citrix.com>

GitHub: closes #14 on xenserver/xencert