https://bugzilla.redhat.com/show_bug.cgi?id=1113860

We've always done that. Well, until 990e46c4. Point is, if we don't
format model, we may lose a domain on libvirtd restart. If the
seclabel is implicit however, we should skip it's formatting.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>