Service dogs and "Assistance Animals" (basically an umbrella term from the FHA that includes both Service dogs and ESAs) are exempted from all breed bans (including insurance). Basically put, just as you are forced to accept it, so are they. IF the dog were to bite someone (while Service dogs are highly trained and this is unlikely, ESAs are not), the owner of the dog would have the final liablity for it anyway (so your insurance company would just pass the liability onto them anyway, but in the event that you were directly sued, the insurance company would have to cover it, as it was an "assistance animal").
Now, that being said, the "gotcha" that could come up (although this is pretty much just academic at this point, but in theory), is what if the insurance company can prove that it wasn't an "assistance animal" after all? That's one of the dangers of doing AirBNB and having people you don't know stay at your property honestly, but in the end, it's academic because the ultimate liability would be on the animals owner anyway.