HONG KONG — The boom in initial public offerings in Hong Kong has offered a long-awaited release valve for private equity firms sitting on aging China portfolios, top fund executives said at an industry panel Tuesday. After years of muted dealmaking and frozen exits, the opportunity to list in Hong Kong at attractive valuations has lifted sentiment, with companies raising $18.2 billion via IPOs this year as of October, putting the financial hub on track to become the world's largest listing