Mac: Allow selecting bundles in the disk prefs

Users should be able to select sparsebundles in the disk prefs, but should
still be able to navigate inside .sheepvm bundles.

See https://github.com/cebix/macemu/pull/25#issuecomment-15900806
Merge pull request #46 from vasi/launcher-add-bundle

Mac: Allow selecting bundles in the disk prefs