The most literal sense of the expression "hog heaven" is a filthy pig sty with plenty of fresh slop on which to feed. Thus the pigs would be just about as happy as they could be.
People extend this idea to other, perhaps less filthy, situations. I've often heard people say someone was in hog heaven for just about any serendipitous occurrence.
So he walks in and finds a bathtub full of whiskey. Man, he was in hog heaven!