SMP issue

I don't think we have an issue for this yet, so I'm opening one now to track this issue.
Currently there are issues running rusty-demo with multiple cores, where sometimes it takes a long time to complete the demo.

## Jobs failed due to SMP timeout (May 20 - June 06):
- https://github.com/hermitcore/libhermit-rs/runs/726877033
- https://github.com/hermitcore/libhermit-rs/runs/725684756
- https://github.com/hermitcore/libhermit-rs/runs/725658934
- https://github.com/hermitcore/libhermit-rs/runs/696784822
- https://github.com/hermitcore/libhermit-rs/runs/695711886
- https://github.com/hermitcore/rusty-hermit/runs/732583279

## Possibly related Job logs:
- [```thread 'main' panicked at 'Trying to get the scheduler for core 1, but it isn't available', src/scheduler/mod.rs:530:9```](https://travis-ci.com/github/hermitcore/rusty-hermit/jobs/343843527#L976) for https://github.com/hermitcore/rusty-hermit/commit/64ec710f8a683eeb0a5a6d3baa591fad794e4fe5