Package qemuguesttools implements the command that runs inside a QEMU VM. These guest tools are responsible for fetching and executing the task command, as well as posting the log from the task command to the meta-data service.
The guest tools are also responsible for polling the meta-data service for actions to do like list-folder, get-artifact or execute a new shell.
The guest tools is pretty much the only way taskcluster-worker can talk to the guest virtual machine. As you can't execute processes inside a virtual machine without SSH'ing into it or something. That something is these guest tools.