I'm sure it's the same as "Ciao." WordReference doesn't seem to recognized "Ciaou," but Google Translate translates it as "Hello." It's possible that Hemingway might have misspelled it and nobody checked or thought it was important. It's also possible that it has more than one spelling in Italian, just as in English we have the variant spellings "Goodbye," "Good-bye," and "Good-by."