exec: RCUify AddressSpaceDispatch

Note that even after this patch, most callers of address_space_*
functions must still be under the big QEMU lock, otherwise the memory
region returned by address_space_translate can disappear as soon as
address_space_translate returns.  This will be fixed in the next part
of this series.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/ppc/spapr_iommu.c

@@ -59,6 +59,7 @@ static sPAPRTCETable *spapr_tce_find_by_liobn(uint32_t liobn)
     return NULL;
 }
 
+/* Called from RCU critical section */
 static IOMMUTLBEntry spapr_tce_translate_iommu(MemoryRegion *iommu, hwaddr addr,
                                                bool is_write)
 {