How is choosen the master or primary slot of the cluster?

It doesn't matter which blade is chosen as the primary for either the vCMP host or vCMP guest. If you had more blades and more guests its entirely possible to have the primary of all of them be different. As to how that decision is made, I have not found documentation on that, though through observations it appears it generally chooses the first available slot but don't quote me on that. It may choose the least busy, but I honestly couldn't say for sure.