After the previous patch, TLBs will be flushed on every change to
the memory mapping. This patch augments that with synchronization
of the MemoryRegionSections referred to in the iotlb array.
With this change, it is guaranteed that iotlb_to_region will access
the correct memory map, even once the TLB will be accessed outside
the BQL.
Backports commit 9d82b5a792236db31a75b9db5c93af69ac07c7c5 from qemu
