cli: Display option 'device vg' only when bd backend enabled

Change-Id: If61c237948f51d48305f4897b3f226eead10bae8
BUG: 912997
Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Reviewed-on: http://review.gluster.org/4555
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>