i440fx: print an error message if user tries to enable iommu

There's no indication of any sort that i440fx doesn't support
"iommu=on"

Reviewed-by: Eric Blake <eblake@redhat.com>
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>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Bandan Das <bsd@redhat.com>