machine: remove iommu property

Since iommu devices can be created with '-device' there is
no need to keep iommu as machine and mch property.

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>