https://bugzilla.redhat.com/show_bug.cgi?id=1287940
I'm not sure what I saw that made me think the entire drivers/md tree was included. I deleted the first VM that I used to investigate the problem, so I can't look at the shell history to figure that one out. In any case, that's not what dracut does.
I filed a bug suggesting that dm-snapshot should always be included for systems on LVM.