Initial implementation on unified memory access

src/common/host_memory.h

@@ -62,6 +62,10 @@ public:
         return backing_base;
     }
 
+    [[nodiscard]] size_t BackingSize() const noexcept {
+        return backing_size;
+    }
+
     [[nodiscard]] u8* VirtualBasePointer() noexcept {
         return virtual_base;
     }