memory: convert memory_region_destroy to object_unparent

Explicitly call object_unparent in the few places where we
will re-create the memory region.  If the memory region is
simply being destroyed as part of device teardown, let QOM
handle it.

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

hw/mips/gt64xxx_pci.c

@@ -297,7 +297,7 @@ static void gt64120_pci_mapping(GT64120State *s)
       if (s->PCI0IO_length)
       {
           memory_region_del_subregion(get_system_memory(), &s->PCI0IO_mem);
-          memory_region_destroy(&s->PCI0IO_mem);
+          object_unparent(OBJECT(&s->PCI0IO_mem));
       }
       /* Map new IO address */
       s->PCI0IO_start = s->regs[GT_PCI0IOLD] << 21;