Prepare for extracting the IOMMU part to a separate function.  Mostly
cosmetic; the only semantic change is that, if there is more than one
cascaded IOMMU and the second one fails to translate, *plen_out is now
adjusted according to the page mask of the first IOMMU.

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