LoongArch: KVM: Add implementation with IOCSR_IPI_SET

LoongArch: KVM: Add implementation with IOCSR_IPI_SET

IPI IOCSR register IOCSR_IPI_SET can send ipi interrupt to other vCPUs,
but it can also send an interrupt to vCPU itself. Indeed there are such
operations on Linux as arch_irq_work_raise() which will send ipi message
to vCPU itself.

Here add implementation of write operation with IOCSR_IPI_SET register.

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