RFR (XS): 8059527: Disallow ParallelGCThreads=0 for G1

[Marcus Larsson]

Hi,

Can I have reviews for the following small patch disallowing 
ParallelGCThreads to be 0 when G1 is used? Also included a small test to 
verify that the VM will not start if the count is 0 for parallel 
collectors (G1, CMS+ParNew and ParallelGC).

Webrev:
http://cr.openjdk.java.net/~mlarsson/8059527/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8059527

Testing:
jprt+jtreg

Thanks,
Marcus