hw/i386: Fix IVHD entry length for AMD IOMMU

Counting from the IVHD ID field to the all-devices entry, we have 28
bytes, not 36.

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>