When has this to happen?
A largely Italian POD might work in 1848 (although it would affect Austria pretty quickly*) but after Sardinian involvement in the Crimean War, any POD that thwarts Risorgimento almost has to have quite immediate international ramifications.
The gist of it is that, while united Italy was mostly the working of Italians themselves, with a "little" help from their friends (Napoleon) the unification was an inherently an international phenomenon since before it began (at least since the Crimean War I would say). If you are fine with a pre-1855 POD, it is doable, after that, it gets complicated.
* You could go for Vittorio Emanuele being an asshole and going along with Austrian demands to abolish the Constitution and play the good guardian of the old Absolutist order. Even better, you could go for Carlo Alberto seeing that the 1848 had little future (hard with what was going on in France) and deciding that siding with the old order was the best bet. He was quite the opportunist, although he had some very vague but at some sincere sort-of-liberal leaning, so it is conceivable that he would stick to absolutism and refused to go to war with Austria. This is a 1848 PoD, with immediate big ramifications, but it is Italian and doesn't touch anything outside to be done.
BTW, failed Italian unification means no recognizable WWI at all.