This moves the EBUSY detection to devmapper.go, and then returns
a real ErrBusy that deviceset uses.
Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Upstream-commit: 586a511cb5e47127a8fd173159fcd43e1b823fe7
Component: engine