Support optionally forwarding agent and systemd journal logs from within VM to the host for debugging

It's often very helpful for debugging to have logs from the agent process running inside the VM. Getting those logs is not straightforward as by default they will stay inside the VM.

One quick hack to get them is [this change](https://github.com/sipsma/firecracker-containerd/commit/2be7fb3c969438e16fca23b86ff151d1ea3af08d), which forwards the agent logs over `/dev/console` to the runtime process on the host, which in turn forwards them to containerd logs.

That approach can't be merged into master though as it results in a lot of noise in the containerd logs and could possibly lead to heavy use of `/dev/console`, which has been known to result in performance issues in the past. A mergable solution to this would need to deal with those issues; some ideas to explore are only forwarding them in a debug mode and making the destination of the logs configurable (could be containerd logs, could be a file, etc.).