memory: add ref/unref calls

Add ref/unref calls at the following places:

- places where memory regions are stashed by a listener and
  used outside the BQL (including in Xen or KVM).

- memory_region_find callsites

- creation of aliases and containers (only the aliased/contained
  region gets a reference to avoid loops)

- around calls to del_subregion/add_subregion, where the region
  could disappear after the first call

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

xen-all.c

@@ -459,6 +459,7 @@ static void xen_set_memory(struct MemoryListener *listener,
 static void xen_region_add(MemoryListener *listener,
                            MemoryRegionSection *section)
 {
+    memory_region_ref(section->mr);
     xen_set_memory(listener, section, true);
 }
 
@@ -466,6 +467,7 @@ static void xen_region_del(MemoryListener *listener,
                            MemoryRegionSection *section)
 {
     xen_set_memory(listener, section, false);
+    memory_region_unref(section->mr);
 }
 
 static void xen_sync_dirty_bitmap(XenIOState *state,