This component, known as crosvm, runs untrusted operating systems along with virtualized devices. No actual hardware is emulated. This only runs VMs through the Linux's KVM interface. What makes crosvm unique is a focus on safety within the programming language and a sandbox around the virtual devices to protect the kernel from attack in case of an exploit in the devices.
The crosvm source code is organized into crates, each with their own unit tests. These crates are:
kernel_loaderLoads elf64 kernel files to a slice of memory.
kvm_syslow-level (mostly) auto-generated structures and constants for using KVM
kvmunsafe, low-level wrapper code for using kvm_sys
crosvmthe top-level binary front-end for using crosvm
x86_64Support code specific to 64 bit intel machines.
Currently there is no front-end, so the best you can do is run
cargo test in each crate.