Only create 4 ports on storage controller

As noted in this comment:

https://github.com/boot2docker/boot2docker-cli/commit/9935fda0d9ad944f8f5cabba23c21655a65cd298#commitcomment-8148801

This speeds up boot by ~10 seconds.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>