Carnival Corp. reported a preliminary quarterly loss of more than $4 billion, its largest ever, as the Covid-19 pandemic has idled ships, and it warned that it could breach a loan agreement in a prolonged sailing pause. The cruise operator on Thursday also said it expects to book a loss for the rest of the fiscal year. It has extended sailing pauses to the end of the year on its other units as it faces differing cruising restrictions in various countries. …