LoongArch: KVM: Skip PMU checking on vCPU context switch

LoongArch: KVM: Skip PMU checking on vCPU context switch

PMU hardware about VM is switched on VM exit to host rather than vCPU
context sched off, PMU is checked and restored on return to VM. It is
not necessary to check PMU on vCPU context sched on callback, since the
request is made on the VM exit entry or VM PMU CSR access abort routine
already.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>