LoongArch: KVM: Add PV steal time support in host side

LoongArch: KVM: Add PV steal time support in host side

Add ParaVirt steal time feature in host side, VM can search supported
features provided by KVM hypervisor, a feature KVM_FEATURE_STEAL_TIME
is added here. Like x86, steal time structure is saved in guest memory,
one hypercall function KVM_HCALL_FUNC_NOTIFY is added to notify KVM to
enable this feature.

One CPU attr ioctl command KVM_LOONGARCH_VCPU_PVTIME_CTRL is added to
save and restore the base address of steal time structure when a VM is
migrated.

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