LoongArch: KVM: Implement vcpu timer operations

LoongArch: KVM: Implement vcpu timer operations

Implement LoongArch vcpu timer operations such as init kvm timer,
acquire kvm timer, save kvm timer and restore kvm timer. When vcpu
exit, we use kvm soft timer to emulate hardware timer. If timeout
happens, the vcpu timer interrupt will be set and it is going to be
handled at vcpu next entrance.

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>