security_dac: rework callback parameter passing
    
    Currently, the DAC security driver passes callback data as
    
        void params[2];
        params[0] = mgr;
        params[1] = def;
    
    Clean this up by defining a structure for passing the callback
    data.  Moreover, there's no need to pass the whole virDomainDef
    in the callback as the only thing needed in the callbacks is
    virSecurityLabelDefPtr.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>