security_dac: rework callback parameter passing

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>