hw/iommu: enable iommu with -device

Use the standard '-device intel-iommu' to create the IOMMU device.
The legacy '-machine,iommu=on' can still be used.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>