Devmapper: Mount images readonly when calculating changes

There is no need to have this be writable, and there is a chance
that e.g. atime updates will cause writes to the image which is
bad for disk use wrt sharing between all containers.
Merge remote-tracking branch 'alexlarsson/dm-readonly-images' into dm

Conflicts:
	devmapper/deviceset_devmapper.go
devmapper: rename deviceset_devmapper.go to deviceset.go
Increase readbility of unit tests by using mkRuntime everywhere
hack: encode the name of the current test in temporary directories, for easier tracking
devmapper: the tests are fast enough that we no longer need a workaround to avoid timeouts
Merge branch 'dm' of ssh://github.com/dotcloud/docker into dm