Virtal File System: Write Support

# What
Add write support to our virtual file system.

# Why
#126 only adds a read-only virtual file system that is populated by the host using a pre-packaged TAR file. There are however good reasons to support writes as well:

- #134
- tempfile support for guests
- #135

# How
Extend the current implementation with whatever we need for writes. Be careful to not introduce DoS vectors.