Kernel: Simplify the way PhysicalRegions are constructed

Instead of creating a PhysicalRegion and then expanding it over and
over as we traverse the memory map on boot, we now compute the final
size of the contiguous physical range up front, and *then* create a
PhysicalRegion object.

Kernel/VM/PhysicalRegion.h

@@ -25,7 +25,6 @@ public:
 
     ~PhysicalRegion();
 
-    void expand(PhysicalAddress lower, PhysicalAddress upper);
     unsigned finalize_capacity();
     void initialize_zones();