memory: avoid unnecessary object_ref/unref

For the common case of DMA into non-hotplugged RAM, it is unnecessary
but expensive to do object_ref/unref.  Add back an owner field to
MemoryRegion, so that these memory regions can skip the reference
counting.

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

include/exec/memory.h

@@ -172,6 +172,7 @@ struct MemoryRegion {
     bool global_locking;
     uint8_t dirty_log_mask;
     ram_addr_t ram_addr;
+    Object *owner;
     const MemoryRegionIOMMUOps *iommu_ops;
 
     const MemoryRegionOps *ops;