Under the rules as written, chemically induced unconsciousness is not magical. For example, the poison Blue Whinnis renders those who fail their saves unconscious. Similarly, the drug Dreamtime Tea knocks out the imbiber with no save against the unconsciousness (though one is allowed a save to avoid addiction to the tea).
The elven immunity explicitly says "Elves are immune to magic sleep effects". Neither of the two samples above are magic, and so the immunity does not apply.
In the case of the dwarven racial trait "Hardy", the +2 on saves versus poison would apply to the blue whinnis, but not to the dreamtime tea. The former is a poison, the latter is a drug. Although "poison" and "drug" are logically both chemicals, they are technically governed by distinct rules systems.
If you are planning to knock out your PCs in order to initiate a scenario, be advised that some players find this upsetting. I advise using this tactic sparingly, only with good reason, and making sure your PCs are quickly given the opportunity to take control of their own actions once again.
