Was disarming (banning the owning/carrying of weapons) the commoner-class an actually policy in the middle ages? I see it in other fantasy works, and it seems strange to disarm men you are very likely to be calling on to fight for you.

Depends very much on the time and place, and whether the establishment saw armed peasants as a military asset or a threat to the social hierarchy.. 

For example, the English monarchy did the opposite – the Assize of Arms of 1182 and 1252 made it a legal obligation for all adult men to own and maintain arms and armor (with the longbow as a minimum requirement by the latter law) so that they could be called up to fight for the King (with the exception of Jews, who were commanded to sell their arms and armor), and Edward III’s decree of 1363 required all adult men to practice archery. 

By contrast, in the 15th century, it was illegal for peasants to carry weapons without royal permission in Sweden, following a revolt against the Union of Kalmar. In Germany following the Great Peasants War, many jurisdictions banned large numbers of former rebels (sometimes entire communities) from owning weapons. In Japan, the famous “sword hunts” of Oda Nobunaga and Toyotomi Hideyoshi disarmed the peasantry, especially the largely peasant warrior-monks who had so threatened the power of the samurai.