LoongArch: KVM: Use kvm_get_vcpu_by_id() instead of kvm_get_vcpu()

Since using kvm_get_vcpu() may fail to retrieve the vCPU context,
kvm_get_vcpu_by_id() should be used instead.

Fixes: 8e305426 ("LoongArch: KVM: Add IPI user mode read and write function")
Fixes: 3956a52b ("LoongArch: KVM: Add EIOINTC read and write functions")
Reviewed-by: Yanteng Si <siyanteng@cqsoftware.com.cm>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>