Handling a qvm termination

The vdev's control function is implemented as follows:

qvmterm_control(vdev_t *vdp, unsigned const ctrl, const char *const arg) {
	struct term_state *state = vdp->v_device;

	switch (ctrl) {
		
		case VDEV_CTRL_TERMINATE:
		// The qvm is terminating.
		// Do any cleanup necessary for the vdev: end any worker threads
		// it created, close its connections to backend drivers, etc.
		// In this example, one worker thread was created by this vdev from
		// the vdev_thread_create() function when the guest was configured
		// (VDEV_CTRL_GUEST_CONFIGURED message block).
		// End the thread.
		pthread_cancel(state->thread); pthread_join(state->thread, NULL);
		break;
	}

The vdev populates its factory structure (qvmterm_factory) with the control function, and the safety variant requirement, and nothing else:

qvmterm_register(void) {
	static struct vdev_factory qvmterm_factory = {
		.next = NULL, // patched
		.control = qvmterm_control,
		.vread = NULL,
		.vwrite = NULL,
		.option_list = NULL,
		.name = NULL, // patched
		.factory_flags = VFF_NONE,
		.acc_sizes = 1u << 1,
		// Make sure we have local data space for our internal structure.
		.extra_space = sizeof(struct qvmterm_state),
		// For the safety version of vdev-qvmterm, we require safety.
		// For the non-safety version we do not require safety.
		//.safety = VDEV_SAFETY_SELECTED,
	};
	vdev_register_factory(&qvmterm_factory, QVM_VDEV_ABI);
}