Make VirtualBox driver build on OpenBSD

[unkaktus]

No description provided.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "openbsd-virtualbox" git@github.com:nogoegst/machine.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[dgageot]

@nogoegst You need to amend the commit so that the code is formatted according to the project rules. See https://github.com/docker/machine/blob/master/CONTRIBUTING.md#tests-and-validation

[dgageot]

LGTM