Austria would need to win the war, plain and simple, for that to happen.
Thus, there's no plausible way Sardinia gets away with the Duchies, especially not Tuscany, though depending on the specifics, Parma might go.
But if Austria is in the military position to retain Lombardy, they would want to crush Sardinia as hard as they can, so probably not just stopping them from any annexation, but pressing on the military advantage as far as possible and end the Sardinian threat once and for all, by dictating a punitive peace: Sesia river border, maybe even Western Liguria divided between Parma and Modena, quite possibly France is still given Nice and Savoy (and perhaps also Tenda, or even Aosta though the latter seems extreme) anyway (to appease them and drive a wedge between Paris and Turin). This is of course the radical, best-case scenario for Vienna, not an extremely likely one.
A more limited Austrian victory would lead to plain
status quo ante. Still, the Duchies are not likely to be touched at all, being kept going by Austrian life support (which a victorious Austria would be in the position to provide). As stated, Parma might be still annexed by Sardinia, all or in part, in case of stalemate in last part of the campaign, but I don't think it very likely assuming Austria repelled the invasion of Lombardy.
In any event, no Italian unification at all is happening here.
There are scenarios on this board where Sardinia manages to unify all of Italy except Lombardy-Venetia (in DoD for example) but they are based on different earlier PODs (and I don't think they are very plausible anyway; with all respect to
@Jared 's outstanding work, I felt the treatment of Italy among the weakest parts of DoD, though I understand the narrative logic there).
IOTL, however, Lombardy was key. It was, by far, the most developed region in Italy economically (which largely still is) and even more importantly, it was the most active culturally, literally the pivot of the entire cultural side of the Risorgimento (more specifically, Milan was). The area had a significant section of the populace highly discontented about Austrian rule, and the role of this group in the development of Italian Nationalism was critical. Many were exiled in Piedmont and able to influence Turin's policy to a point. Lombardy was also the primary target of Sardinian ambitions.
Stopping Sardinia there would mean no Italian unification forthcoming, certainly not one led and managed by Turin. Cavour would be discredited by a defeat and would certainly lose office. This does not mean Italian Unification in some form will not happen at all, the momentum behind it was quite huge by that point and the old order is hardly lasting forever.
Finally, the OP's premise is low-probability. It is hard to see Austria winning the war outright. First thing, Gyulai needs to be replaced by some Austrian competent commander for the theatre. Not sure it would be enough against France and Sardinia combined, considering that the Austrian had no military edge except fighting on the defensive, and the civilian population in the area was largely very hostile to them.