target/loongarch: fix vcpu reset command word issue

target/loongarch: fix vcpu reset command word issue

When the KVM_REG_LOONGARCH_VCPU_RESET command word
is sent to the kernel through the kvm_set_one_reg interface,
the parameter source needs to be a legal address,
otherwise the kernel will return an error and the command word
will fail to be sent.

Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Bibo Mao <maobibo@loongson.cn>