It is enhancement which allows to unpack image into containerd
snapshotter storage by `--output type=image,<.>=<.>,unpack=true`.
In order to support this feature, we needs to extend the Snapshotter
witwh `Name() string` function. Because we needs to set gc label for
snapshotter which need snapshotter name.
fix: #908
Signed-off-by: Wei Fu <fuweid89@gmail.com>