MEM: Make port proxies use references rather than pointers

This patch is adding a clearer design intent to all objects that would
not be complete without a port proxy by making the proxies members
rathen than dynamically allocated. In essence, if NULL would not be a
valid value for the proxy, then we avoid using a pointer to make this
clear.

The same approach is used for the methods using these proxies, such as
loadSections, that now use references rather than pointers to better
reflect the fact that NULL would not be an acceptable value (in fact
the code would break and that is how this patch started out).

Overall the concept of "using a reference to express unconditional
composition where a NULL pointer is never valid" could be done on a
much broader scale throughout the code base, but for now it is only
done in the locations affected by the proxies.

src/dev/simple_disk.cc

@@ -70,7 +70,7 @@ SimpleDisk::read(Addr addr, baddr_t block, int count) const
     for (int i = 0, j = 0; i < count; i += SectorSize, j++)
         image->read(data + i, block + j);
 
-    system->physProxy->writeBlob(addr, data, count);
+    system->physProxy.writeBlob(addr, data, count);
 
     DPRINTF(SimpleDisk, "read  block=%#x len=%d\n", (uint64_t)block, count);
     DDUMP(SimpleDiskData, data, count);