Please review fix for 7196103: NPG: Unable to allocate bit map for parallel garbage collection for the requested heap size

[Coleen Phillimore]

Summary: Don't allocate huge class metaspace size by default on x64

This was preventing other mmap regions from being initialized on windows 
64.  I also added some checking around the ClassMetaspaceSize option.  
1*M seems to be minimal for startup size

open webrev at http://cr.openjdk.java.net/~coleenp/7196103/

Thanks,
Coleen