I'm not too sure about it's "lost" status - I've had the music for it for years (published, not downloaded). It's been recorded before too, along with his variations on a theme by Chopin.
The trace of the score was lost for more than a century and was found only several years ago in the archive of the Glinka museum, in the collection of professor Siloti, Rachmaninoff's professor.
Alexander Rachmaninoff, the grand-son of the great composer and the keeper of the Rachmaninoff family heritage, entrusted Denis Matsuev with the first record of the archival score. The record was made in May 2007 on the villa "Senar" in Switzerland, where Rachmaninoff lived in years 1929-1940.
The Suite in D minor is a recently discovered work, not the Corelli Variations in D minor. The two compositions are in no way related.