memory: add address_space_access_valid

The old-style IOMMU lets you check whether an access is valid in a
given DMAContext.  There is no equivalent for AddressSpace in the
memory API, implement it with a lookup of the dispatch tree.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

dma-helpers.c

@@ -298,6 +298,11 @@ bool iommu_dma_memory_valid(DMAContext *dma, dma_addr_t addr, dma_addr_t len,
             plen = len;
         }
 
+        if (!address_space_access_valid(dma->as, paddr, len,
+                                        dir == DMA_DIRECTION_FROM_DEVICE)) {
+            return false;
+        }
+
         len -= plen;
         addr += plen;
     }