Allow exposing the partitions tempfs to the outside

### Introduction

There should be a mode for the hypervisor which allows partitions the creation of files. Each `/` as perceived by the partition should map to a folder in the host environment, which might be determined by the CLI flag which enables the behavior.

### Behavior

- Given the flag `--partition-fs $PWD/part-fs` is set while launching the hypervisor
- When the partition `part_1` creates a file under `/my-file` (all as seen from the mnt namespace of `part_1`)
- Then this file should be visible on the host side (in the root mnt namespace) as `$PWD/part-fs/part_1/my-file`

### Why?

I would like to use the hypervisor for instrumentation based coverage measurements. Most tools (like llvm-cov) rely on writing the coverage traces during runtime of the application to a file. If some sort of filesystem shared with host environment and the partitions is available, doing the instrumentation based coverage would be easy.

### Caveat:

- What if the hypervisor quits, wouldn't that delete the tempfs of the partitions?
- on quit, the hypervisor could move the files from all tempfs of the partitions to the actual destination
- How does this interact with the FS API mentioned in A653P2?
- ???