You would use "Single instrument with piano piece" or, if a piece is a sonata, "Sonata (piano with 1 instrument). Don't worry too much if you didn't choose a correct one -- it can be changed after you submit it. I regularly correct small things like that.
If a flute player wants to search for something like this, he can go to the search box and type in:
"Single instrument with piano piece" flute
and they will all come up.
[Edit] I have corrected the genre now, but I am going to download this at work next week. Depending on which composers are in the book, I may change the instrumentation to harpsichord, since the piano was a later 1700s instrument.