LoongArch: KVM: Move host CSR_GSTAT save and restore in context switch

LoongArch: KVM: Move host CSR_GSTAT save and restore in context switch

CSR register LOONGARCH_CSR_GSTAT stores guest VMID information. With
existing implementation method, VMID is per vCPU, similar with ASID in
kernel. LOONGARCH_CSR_GSTAT is written at VM entry even if VMID is not
changed.

Here move LOONGARCH_CSR_GSTAT save/restore in vCPU context switch, and
update LOONGARCH_CSR_GSTAT only when VMID is updated at VM entry. At
most time VM enter/exit is much more frequent than vCPU thread context
switch.

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