Save restore device Id: issue #8978

Fixes #8978

During testing of docker with device mapper backend, I often noticed bunch of warnings on console and often thought that if something is wrong with my docker setup. It turned out that docker does not keep track of already used device ids.  This patch series should make docker little bit smarter about it.

More details are in issue and patches. Please review and pull.

Thanks
Vivek
@rhvgoyal it's not about the immutability of the string, its about the access of the path. Like

```golang
var devicesetMetadataFilename = "deviceset-metadata"
func (devices *DeviceSet) deviceSetMetaFile() string {
	return path.Join(devices.metadataDir(), devicesetMetadataFilename)
}
```
for reuse