I believe you would just have ownership of the creature, as if you had cast a mind control on it yourself.
Mind Control, as per its last line, cannot be moved to a new target so it cant leave its current "host" so to speak. But since you're both Forcemasters (or any Mind Mage for the purpose of MC) you are legally allowed to control the enchantment
so, steal grants you control of Mind Control which grants you control of the attached creature, which was yours originally. This would also prevent the opponent from playing Mind Control on him again, since he cannot have 2 copies of the same spell attached.
I *think* thats how it would play out.