virDomainReboot: Document that migration might be unsafe
    
    https://bugzilla.redhat.com/show_bug.cgi?id=744967
    
    If a domain is rebooting and a migrate API is called meanwhile we would
    have to transfer the fakeReboot attribute to the destination in order to
    prevent domain doing plain shutdown over there. We shouldn't try to do
    anything clever about it other than documenting this as a known
    limitation.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>