devicemapper: fix zero-sized field access

Fixes: #15279

Due to
https://github.com/golang/go/commit/7904946eeb35faece61bbf6f5b3cc8be2f519c17
the devices field is dropped.

This solution works on go1.4 and go1.5

Signed-off-by: Vincent Batts <vbatts@redhat.com>