virSystemdCreateMachine: Set dependencies for slices
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1031696
    
    When creating a new domain, we let systemd know about it by calling
    CreateMachine() function via dbus. Systemd then creates a scope and
    places domain into it. However, later when the host is shutting
    down, systemd computes the shutdown order to see what processes can
    be shut down in parallel. And since we were not setting
    dependencies at all, the slices (and thus domains) were most likely
    killed before libvirt-guests.service. So user domains that had to
    be saved, shut off, whatever were in fact killed.  This problem can
    be solved by letting systemd know that scopes we're creating must
    not be killed before libvirt-guests.service.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>