CA-90845 : XenCert functional iscsi test fails to detect LUNs and passes.

Corrected device path, raise error when LUN not present, also increased
sector count while calculating the estimate.

Signed-off-by: Sugandha Aggarwal <sugandha.aggarwal@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #12 on xenserver/xencert