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
Hmm..., I am not sure I understand the concern here. My understanding is that strings are immutable in go language. So one can not modify the string returned by deviceSetMetaFile(). Please give little more details.