spapr: make IOMMU translation go through IOMMUTLBEntry

The next step is to introduce the translation code that will be used for
IOMMU MemoryRegions, but still do the actual translation in a DMAContext.

Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>