dummy driver: Use cp --reflink=auto to copy directories

On systems that supports reflinking (i.e. btrfs) this means the dummy
backend is much faster at copying files and will be sharing file data in
a CoW fashion.

On my (btrfs) system this makes "docker run ubuntu echo hello world" go
from about 3 seconds to about 1 second. Not instant, but clearly better.

cp --reflink=auto is availible since coreutils 7.5 (around 2009), so this
seems pretty ok to rely on.

cp is also better at preserving file metadata than tar, so for instance
it will copy xattrs.
Merge pull request #46 from alexlarsson/dm-plugin-dummy-use-cp

Dummy driver: Use cp --reflink=auto to copy directories