TL;DR, if I understand how Tor relays work, Unbound (or any local DNS server) should see a request for
example.com coming from localhost or 127.0.0.1. It answers the request, stores it in cache just in case, rinse and repeat. The machine running the exit relay is the one that makes the DNS request, so the only thing you'd get from looking at the DNS cache would be a "Top 100 Websites This Tor Relay Visits" sort of list.
From what I could find, a DNS cache contains the hostname and its associated IP address, nothing more. From what I understand, even if a DNS cache saved the source of the request, it should save "127.0.0.1" or "localhost" as the source, since exit nodes are the source of the request, and simply forward the response back to the client.
I couldn't find anything specific about Unbound, but it seems like there isn't a proper way to read the DNS cache anyway unless you can somehow decode the binary file. I suppose if you know the specific cache file, you could copy it to a different machine with Unbound installed, and possibly extract data from that, but this theory assumes the cache is saved to the hard drive, and it's probably only stored in RAM.