SW timer is enabled when vcpu thread is scheduled out, and it is to wake
up vcpu from blocked queue. If vcpu thread is scheduled out but is not
blocked, such as it is preempted by other threads, it is not necessary
to enable SW timer. Since vcpu thread is still on running queue if it is
preempted and SW timer is only to wake up vcpu on blocking queue, so SW
timer is not useful in this situation.

This patch enables SW timer only when vcpu is scheduled out and is
blocking.
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>