Host filesystem setup to prepare for running a Cuttlefish device.

Supports rewriting super.img, boot.img, and initramfs.img based on launch_cvd arguments. In a multi-device configuration, there is only one assemble_cvd invocation, but multiple run_cvd invocations, one per device.

The disk files produced by the Android build system are arranged into a “composite disk” and protected with a read-on-write qcow2 overlay.