RFR 8061616: HotspotDiagnosticMXBean.getVMOption() throws IllegalArgumentException for flags of type double

[Jaroslav Bachorik]

Please review the following change

Issue  : https://bugs.openjdk.java.net/browse/JDK-8061616
Webrev :
   (jdk): http://cr.openjdk.java.net/~jbachorik/8061616/jdk/webrev.00/
   (hotspot): 
http://cr.openjdk.java.net/~jbachorik/8061616/hotspot/webrev.00/

Currently the double values for VM flags are not supported in 
HotspotDiagnosticMXBean. This change is about implementing this support 
similarly as it is done for eg. long values.

Thanks,

-JB-