Avoid extra mount/unmount during build

CmdRun() calls first run() and then wait() to wait for it to exit,
then it runs commit(). The run command will mount the container and
the container exiting will unmount it. Then the commit will
immediately mount it again to do a diff.

This seems minor, but this is actually problematic, as the Get/Put
pair will create a spurious mount/unmount cycle that is not needed and
slows things down. Additionally it will create a supurious
devicemapper activate/deactivate cycle that causes races with udev as
seen in https://github.com/dotcloud/docker/issues/4036.

To ensure that we only unmount once we split up run() into create()
and run() and reference the mount until after the commit().

With this change docker build on devicemapper is now race-free, and
slightly faster.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)