q35: Check propery to determine if iommu is set

The helper function machine_iommu() isn't necesary. We can
directly check for the property.

Signed-off-by: Bandan Das <bsd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Bandan Das <bsd@redhat.com>
