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 its just a simple nit. Line 567 here is not needed, since 568 is the error check of line 566.