RFR (XS) 8152957: Improve specificity of safepoint logging to print safepoint typ

[coleen.phillimore at oracle.com]

Summary: upgrade safepoint begin logs to Info logging, which has the reason.

See bug for sample.

Tested with existing jtreg tests to make sure new logging line doesn't 
break anything.

open webrev at http://cr.openjdk.java.net/~coleenp/8152957.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8152957

Thanks,
Coleen