WIP: Automatically manage FEX rootfs

[alyssarosenzweig]

This adds a mechanism for automatically managing the FEX rootfs, mounting and overlayfs'ing the relevant squashfs files from the host.

Depends on:

• libkrun https://github.com/containers/libkrun/pull/217
• libkrunfw https://github.com/containers/libkrunfw/pull/65