I always assumed that the Cuban police were contracted by Roth or Ola or someone to protect Roth while he was in Cuba. Because of the official function that night with the New Year party and the President being there and everything, the police didnt stay with Roth. Then, they somehow recieved a tip that someone was gonna try to kill Roth (this is where it gets a bit shaky), marched up to the room as quick as they could, and shot Buscetta (the bodyguard) in the room.

As far as who tipped off the cops or how much they knew, I'm not absolutely sure. Although just pondering over it, makes me think maybe it was Fredo. What if, after Mike tells him in the cafe (banana daquiri scene) that Hyman Roth "won't see the new year," he runs to Ola or the cops, tells them about Mike's plans, and then the cops run up the room, catch Buscetta in the act, giving them an easy excuse to kill him, and BAM, its over.

Interesting theory, good question too.