CAR-1744: better truncation logic
CAR-1744: better truncation logic

This handles truncation better in cases where

 * A guest agent fails to trancate properly for us
 * There are minor differences on string lengthing/rounding scheme across components/OSes

Signed-off-by: Zheng Li <34c6fceca75e456f25e7e99531e2425c6c1de443@zheng.li>

djs55/xenopsd,djs55/xenopsd,euanh/xenopsd,thomassa/xenopsd,djs55/xenopsd,euanh/xenopsd,johnelse/xenopsd,johnelse/xenopsd,johnelse/xenopsd,thomassa/xenopsd,euanh/xenopsd,thomassa/xenopsd