RFR(S) JDK-8025526: VirtualSpace should support per-instance disabling of large pages

[Mikael Gerdin]

Hi,

As part of ongoing work in getting Metaspace to properly enforce the 
MaxMetaspaceSize limit [JDK-8024547] we need to extend the VirtualSpace 
class to allow us to set the granularity at which VirtualSpace commits 
pages from the OS.

Summary of changes:
Extend VirtualSpace::initialize to take a parameter for setting the 
maximum commit granularity. The default value 0 keeps the previous 
behavior of using os::page_size_for_region to determine the granularity.
Add unit tests for disabling and enabling large pages both at the 
ReservedSpace and the VirtualSpace level.

Testing:
JPRT run (includes running the new tests)

Webrev: http://cr.openjdk.java.net/~mgerdin/8025526/webrev.0
Bug link: https://bugs.openjdk.java.net/browse/JDK-8025526

/Mikael