hw/loongarch/virt: Fix cpuslot::cpu set at last in virt_cpu_plug()

hw/loongarch/virt: Fix cpuslot::cpu set at last in virt_cpu_plug()

In function virt_cpu_plug(), Object cpuslot::cpu is set at last
only when there is no any error, otherwise it is problematic that
cpuslot::cpu is set in advance however it returns because of error.

Fixes: ab9935d2991e (hw/loongarch/virt: Implement cpu plug interface)
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20250324030145.3037408-2-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>