kvm tools: Exit properly on SMP guests

When shutting down SMP guests only VCPU #0 will receive
a KVM_EXIT_SHUTDOWN. Waiting for all VCPU threads to exit
causes them to hang.

Instead, notify all VCPU threads once VCPU #0 thread is terminated
so they could also stop properly.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>