RFR (S) 8222491: jcmd can fail converting UTF8 output to strings

[Schmelter, Ralf]

Hi,

please review the small fix to jcmd. Instead of trying to convert only 
partially read bytes to Strings by hand, we now use an InputStreamReader 
instead.

webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8222491/webrev.0/
bugreport: https://bugs.openjdk.java.net/browse/JDK-8222491

Best regards,
Ralf