devicemapper has landed, but the TODO hasn't been actioned presumably
because aufs is still preferred over devicemapper when available[1].
Comment updated accordingly.
Citation [1]: 267ca39921/runtime/graphdriver/driver.go (L40-L46)
Docker-DCO-1.1-Signed-off-by: Paul Annesley <paul@annesley.cc> (github: pda)
Upstream-commit: 576278102e0fa9166711f8cf23ec972fcccc085e
Component: engine