memory: Don't call memory_region_update_coalesced_range if nothing changed

With huge number of PCI devices in the system (for example, 200
virtio-blk-pci), this unconditional call can slow down emulation of
irrelevant PCI operations drastically, such as a BAR update on a device
that has no coalescing region. So avoid it.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

memory.c

@@ -1332,6 +1332,7 @@ void memory_region_add_coalescing(MemoryRegion *mr,
 void memory_region_clear_coalescing(MemoryRegion *mr)
 {
     CoalescedMemoryRange *cmr;
+    bool updated = false;
 
     qemu_flush_coalesced_mmio_buffer();
     mr->flush_coalesced_mmio = false;
@@ -1340,8 +1341,12 @@ void memory_region_clear_coalescing(MemoryRegion *mr)
         cmr = QTAILQ_FIRST(&mr->coalesced);
         QTAILQ_REMOVE(&mr->coalesced, cmr, link);
         g_free(cmr);
+        updated = true;
+    }
+
+    if (updated) {
+        memory_region_update_coalesced_range(mr);
     }
-    memory_region_update_coalesced_range(mr);
 }
 
 void memory_region_set_flush_coalesced(MemoryRegion *mr)