graphdriver: Add generic test framework for graph drivers

This adds daemon/graphdriver/graphtest/graphtest which has a few generic tests for all graph drivers, and then uses these from the btrs, devicemapper and vfs backends. There are also some fixes to the existing tests and drivers to make the tests pass.
 
I've not yet added the aufs backend, because i can't test that here atm. It should work though. I also plan to add more tests, but I'll be away for a while, so might as well get this out there before that.