LoongArch: KVM: Implement basic vcpu interfaces

LoongArch: KVM: Implement basic vcpu interfaces

Implement basic vcpu interfaces, including:

1, vcpu create and destroy interface, saving info into vcpu arch
structure such as vcpu exception entrance, vcpu enter guest pointer,
etc. Init vcpu timer and set address translation mode when vcpu create.

2, vcpu run interface, handling mmio, iocsr reading fault and deliver
interrupt, lose fpu before vcpu enter guest.

3, vcpu handle exit interface, getting the exit code by ESTAT register
and using kvm exception vector to handle it.

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>