By Ethan Bilby BRUSSELS (Reuters) - China threatened on Thursday to retaliate if the European Union formally opens an investigation into alleged anti-competitive behavior by Chinese mobile telecom equipment companies. European Trade Commissioner Karel De Gucht said on Wednesday he and fellow commissioners had agreed in principle to open an anti-dumping and anti-subsidy case against China, but would first seek to negotiate a solution with Chinese authorities. Although not mentioned in the statement, EU officials told Reuters the primary targets of the investigation would be world No. ...