Former Sen. John W. Warner of Virginia has died. He was a former Navy secretary who was married to movie star Elizabeth Taylor when he first ran for the senate in 1978, and held onto the seat for 30 years.   Warner died yesterday of heart failure at home in Alexandria with his wife and daughter at his side. Warner was a centrist Republican and powerful chairman of the Senate Armed Services Committee who supported the war in Iraq. He also had an independent streak and drew support from moderates on both sides of America’s political divide. He was 94.