hw/i386: Fix AMDVI GATS and HATS encodings

We support up to 6 levels, but those are encoded as 10b according to the
AMD IOMMU spec (chapter 3.3.1, Extended Feature Register).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>