intel_iommu: cleanup vtd_{do_}iommu_translate()

intel_iommu: cleanup vtd_{do_}iommu_translate()

First, let vtd_do_iommu_translate() return a status, so that we
explicitly knows whether error occured. Meanwhile, we make sure that
IOMMUTLBEntry is filled in in that.

Then, cleanup vtd_iommu_translate a bit. So even with PT we'll get a log
now. Also, remove useless assignments.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>