Can't marshal a Variant value as JSON

[lifubang]

As described by @kolyshkin in opencontainers/runc#3361 (comment), we can't marshal a Variant variable as a json string.

If we use json.Marshal, we will get an empty string(Not an empty string, but an empty JSON like "{}").
If we use Encode in encoding/gob, we will get an error:

Error encoding: gob: type dbus.Variant has no exported fields

Do you want to implement the json interface? Or let others who use the type Variant to implement it in their own projects?