memory: do not give a name to the internal exec.c regions

There is no need to have them visible under /machine.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1801,13 +1801,13 @@ MemoryRegion *iotlb_to_region(AddressSpace *as, hwaddr index)
 
 static void io_mem_init(void)
 {
-    memory_region_init_io(&io_mem_rom, NULL, &unassigned_mem_ops, NULL, "rom", UINT64_MAX);
+    memory_region_init_io(&io_mem_rom, NULL, &unassigned_mem_ops, NULL, NULL, UINT64_MAX);
     memory_region_init_io(&io_mem_unassigned, NULL, &unassigned_mem_ops, NULL,
-                          "unassigned", UINT64_MAX);
+                          NULL, UINT64_MAX);
     memory_region_init_io(&io_mem_notdirty, NULL, &notdirty_mem_ops, NULL,
-                          "notdirty", UINT64_MAX);
+                          NULL, UINT64_MAX);
     memory_region_init_io(&io_mem_watch, NULL, &watch_mem_ops, NULL,
-                          "watch", UINT64_MAX);
+                          NULL, UINT64_MAX);
 }
 
 static void mem_begin(MemoryListener *listener)