This will be used to process IOMMUs in a MemoryRegionCache.  This
includes a small bugfix, in that the returned page_mask is now
correctly -1 if the IOMMU memory region maps the entire address
space directly.  Previously, address_space_get_iotlb_entry would
return ~TARGET_PAGE_MASK.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>