Use memory region map to discover the memory

kernel/platform/i386_pc/sys/mm.h

@@ -49,6 +49,7 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
                                  it would be inefficient to constantly (un)lock. */
 
 #define KERNEL_HEAP_START 0xF0000000
+#define MEM_PROBE_HEAP_START 0xE0000000
 
 struct page_dir {
   volatile uint32_t *pd;