LoongArch: KVM: Implement vcpu world switch

Implement LoongArch vcpu world switch, including vcpu enter guest and
vcpu exit from guest, both operations need to save or restore the host
and guest registers.

Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Tested-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>