devmapper: Take care of some review comments

Took care of some review comments from crosbymichael.

v2:
- Return "err = nil" if file deviceset-metadata file does not exist.
- Use json.Decoder() interface for loading deviceset metadata.

v3:
- Reverted back to json marshal interface in loadDeviceSetMetaData().

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>