memory: MemoryRegion: QOMify

QOMify memory regions as an Object. The former init() and destroy()
routines become instance_init() and instance_finalize() resp.

memory_region_init() is re-implemented to be:
object_initialize() + set fields

memory_region_destroy() is re-implemented to call unparent().

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
[Add newly-created MR as child, unparent on destruction. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/exec.c b/exec.c
index 18d6c35..df4a080 100644
--- a/exec.c
+++ b/exec.c
@@ -883,7 +883,7 @@
 
     if (mr->subpage) {
         subpage_t *subpage = container_of(mr, subpage_t, iomem);
-        memory_region_destroy(&subpage->iomem);
+        object_unref(OBJECT(&subpage->iomem));
         g_free(subpage);
     }
 }
@@ -1768,7 +1768,7 @@
     mmio->as = as;
     mmio->base = base;
     memory_region_init_io(&mmio->iomem, NULL, &subpage_ops, mmio,
-                          "subpage", TARGET_PAGE_SIZE);
+                          NULL, TARGET_PAGE_SIZE);
     mmio->iomem.subpage = true;
 #if defined(DEBUG_SUBPAGE)
     printf("%s: %p base " TARGET_FMT_plx " len %08x\n", __func__,