Moves file_to_disk outside of /tmp

@dcarley suggested that moving the `file_to_disk` VDI outside of /tmp is
probably a neater way of doing this, as `vagrant destroy` might not destroy a
drive outside of the current folder.

This moves it in to the current folder.
Adds *.vdi to .gitignore

Ensures that we don't accidentally commit VDI disk to repo.