By Silvia Aloisi and Pavel Polityuk LVIV (Reuters) – Ukraine faces a possible grain revenue loss of $6 billion as the blockade of its ports by Russian forces prevents it from selling millions of tonnes of wheat and corn that had been earmarked for export by June, a senior industry official said. Countries that rely on imports of Ukrainian wheat – including Egypt, Turkey and Yemen – will need to find alternative supplies, aid agencies have warned. Ukraine, a major producer of grain and oilseeds, exports 98% of its cereals through its ports and only a fraction by rail, where costs are higher. Th…