Device{Attach,Detach}: Document S4 limitations
    
    https://bugzilla.redhat.com/show_bug.cgi?id=808463
    
    Well, libvirt doesn't distinguish between domain poweroff and
    hibernation (S4). It's hard to differentiate these two on a real
    machine anyway. As a result, any device that is hot(un-)plugged is
    lost (appears again) when domain is started again as from our POV
    it is a fresh cold boot. Instead of doing anything wise here, we
    should just document this as known limitation.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>