SystemInformation: Improve Get(Available/Total)VirtualMemory for windows

Currently kwsys can only return ullTotalVirtual, which is not
a very useful value, as memory allocation will fail if we exhaust
the ullTotalPageFile limit.

For example, on a windows workstation, GlobalMemoryStatusEx
returns:
ullTotalVirtual = 128TB
ullTotalPageFile = 16GB

This commit updates the function Get(Available/Total)VirtualMemory to
use page file size.

Change-Id: I461e5f0a975432c31fc7370efe8467c61fea1bb3