By FRANCES ROBLES Diana Sainz and her husband, Andrea Gallina, invested big a decade ago when Cuba made a push toward allowing private entrepreneurship. The couple own two boutique hotels with restaurants in Havana, three small markets and a production center to sell frozen prepared meals. But with fuel running out and long electrical outages a daily occurrence, 90% of this year’s reservations have been canceled. They said they had to lay off dozens of employees. Under intense pressure from...