memory: MemoryRegion: factor out subregion add functionality

Split off the core looping code that actually adds subregions into
it's own fn. This prepares support for Memory Region qomification
where setting the MR address or parent via QOM will back onto this more
minimal function.
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
[Rename new function. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

memory.c

@@ -1423,18 +1423,15 @@ void memory_region_del_eventfd(MemoryRegion *mr,
     memory_region_transaction_commit();
 }
 
-static void memory_region_add_subregion_common(MemoryRegion *mr,
-                                               hwaddr offset,
-                                               MemoryRegion *subregion)
+static void memory_region_update_parent_subregions(MemoryRegion *subregion)
 {
+    hwaddr offset = subregion->addr;
+    MemoryRegion *mr = subregion->parent;
     MemoryRegion *other;
 
     memory_region_transaction_begin();
 
-    assert(!subregion->parent);
     memory_region_ref(subregion);
-    subregion->parent = mr;
-    subregion->addr = offset;
     QTAILQ_FOREACH(other, &mr->subregions, subregions_link) {
         if (subregion->may_overlap || other->may_overlap) {
             continue;
@@ -1468,6 +1465,15 @@ done:
     memory_region_transaction_commit();
 }
 
+static void memory_region_add_subregion_common(MemoryRegion *mr,
+                                               hwaddr offset,
+                                               MemoryRegion *subregion)
+{
+    assert(!subregion->parent);
+    subregion->parent = mr;
+    subregion->addr = offset;
+    memory_region_update_parent_subregions(subregion);
+}
 
 void memory_region_add_subregion(MemoryRegion *mr,
                                  hwaddr offset,