LoongArch: KVM: Restore guest PMU if it is enabled

LoongArch: KVM: Restore guest PMU if it is enabled

On LoongArch system, guest PMU hardware is shared by guest and host but
PMU interrupt is separated. PMU is pass-through to VM, and there is PMU
context switch when exit to host and return to guest.

There is optimiation to check whether PMU is enabled by guest. If not,
it is not necessary to return to guest. However, if it is enabled, PMU
context for guest need switch on. Now KVM_REQ_PMU notification is set
on vCPU context switch, but it is missing if there is no vCPU context
switch while PMU is used by guest VM, so fix it.

Cc: <stable@vger.kernel.org>
Fixes: f4e40ea9f78f ("LoongArch: KVM: Add PMU support for guest")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>