hw/intc/loongarch_ipi: Fix start fail with smp cpu < smp maxcpus on KVM

hw/intc/loongarch_ipi: Fix start fail with smp cpu < smp maxcpus on KVM

QEMU start failed when smp cpu < smp maxcpus , because qemu send a NULL
cpu to KVM, this patch adds a check for kvm_ipi_access_regs() to fix it.

run with '-smp 1,maxcpus=4,sockets=4,cores=1,threads=1'

we got:
Unexpected error in kvm_device_access() at ../accel/kvm/kvm-all.c:3477:
qemu-system-loongarch64: KVM_SET_DEVICE_ATTR failed: Group 1073741825 attr 0x0000000000010000: Invalid argument

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Message-ID: <20250725081213.3867592-1-gaosong@loongson.cn>