LoongArch: KVM: Add PMU support for guest

On LoongArch, the host and guest have their own PMU CSRs registers and
they share PMU hardware resources. A set of PMU CSRs consists of a CTRL
register and a CNTR register. We can set which PMU CSRs are used by the
guest by writing to the GCFG register [24:26] bits.

On KVM side:
- Save the host PMU CSRs into structure kvm_context.
- If the host supports the PMU feature.
  - When entering guest mode, save the host PMU CSRs and restore the guest PMU CSRs.
  - When exiting guest mode, save the guest PMU CSRs and restore the host PMU CSRs.

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