CA-140343: Fix error sign

Incorrect error sign shows "Unknown error 18446744073709551614"
Fixed error sign to get more meaningful error string.

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

GitHub: closes #117 on xapi-project/blktap