Virtal File System: Write Support

[crepererum]

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:

• Extract Root Filesystem Guest-side #134
• tempfile support for guests
• Python Guest-side Dependency Setup #135

How

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