Jethro Tull was an 18th century agriculturalist - it's unlikely any songs he wrote survived and transmogrified into a Californian ballad ;-)
As for the band Jethro Tull - bandleader Ian Anderson wrote a song called We Used to Know which they played on a tour of the States in 1971. The support band on the tour were a newly-formed Eagles... and Hotel California was recorded a year or so later. It appears the songs have the same chord structure... but knowing the music business, if there was any chance of proving the case in court, someone would have tried. So it's probably little more than a coincidence.