https://bugzilla.redhat.com/show_bug.cgi?id=1266088

We are missing this value for cdrom-image device. It seems like
there's no added value to extend this to other types of disk
devices [1].

1: https://www.redhat.com/archives/libvir-list/2016-January/msg01038.htmlSigned-off-by: NMichal Privoznik <mprivozn@redhat.com>