Use system-level values for Windows virtual memory

Submitting this as a draft PR as I believe it will address giampaolo#2074.  I do not have either a C or Python development environment to test these changes, so they are provided in the hopes someone else can test and confirm they fix the issue.

There's probably some room to simplify the code with temporary variables to reduce redundancy.

For background, I have implemented precisely this logic in my own (Java-based) project [here](https://github.com/oshi/oshi/blob/3bb9eafbe2062edad4108b7b12501b1f9be27361/oshi-core/src/main/java/oshi/hardware/platform/windows/WindowsVirtualMemory.java#L110-L118) and [here](https://github.com/oshi/oshi/blob/3bb9eafbe2062edad4108b7b12501b1f9be27361/oshi-core/src/main/java/oshi/hardware/platform/windows/WindowsGlobalMemory.java#L253-L263) following a detailed investigation in oshi/oshi#1182

Signed-off-by: Daniel Widdis <widdis@gmail.com>

psutil/_psutil_windows.c

@@ -610,20 +610,18 @@ psutil_proc_memory_uss(PyObject *self, PyObject *args) {
  */
 static PyObject *
 psutil_virtual_mem(PyObject *self, PyObject *args) {
-    MEMORYSTATUSEX memInfo;
-    memInfo.dwLength = sizeof(MEMORYSTATUSEX);
-
-    if (! GlobalMemoryStatusEx(&memInfo)) {
+    PERFORMANCE_INFORMATION perfInfo;
+    if (! GetPerformanceInfo(&perfInfo, sizeof(PERFORMANCE_INFORMATION))) {
         PyErr_SetFromWindowsErr(0);
         return NULL;
     }
     return Py_BuildValue("(LLLLLL)",
-                         memInfo.ullTotalPhys,      // total
-                         memInfo.ullAvailPhys,      // avail
-                         memInfo.ullTotalPageFile,  // total page file
-                         memInfo.ullAvailPageFile,  // avail page file
-                         memInfo.ullTotalVirtual,   // total virtual
-                         memInfo.ullAvailVirtual);  // avail virtual
+                         perfInfo.PhysicalTotal * perfInfo.PageSize,      // total
+                         perfInfo.PhysicalAvailable * perfInfo.PageSize,  // avail
+                         (perfInfo.CommitLimit - perfInfo.PhysicalTotal) * perfInfo.PageSize, // total page file
+                         (perfInfo.CommitLimit - perfInfo.CommitTotal - perfInfo.PhysicalAvailable) * perfInfo.PageSize,  // avail page file
+                         perfInfo.CommitLimit * perfInfo.PageSize,   // total virtual
+                         (perfInfo.CommitLimit - perfInfo.CommitTotal) * perfInfo.PageSize);  // avail virtual
 }