target/loongarch/kvm: fix uninitialized val and unchecked GET in cpucfg2 check

kvm_check_cpucfg2() discards the return value of KVM_GET_DEVICE_ATTR and uses the local val (the host cpucfg2 mask) without checking whether the read succeeded. val is also declared without an initializer, so on a GET failure env->cpucfg[2] &= val reads an uninitialized value. The &= mask is best-effort feature negotiation: if KVM_HAS_DEVICE_ATTR succeeds, a GET failure is most likely a copy_{from,to}_user issue, not a reason to fail the whole register sync. Check the GET return value, warn and skip the mask on failure (the guest keeps the cpucfg2 it already has), and initialize val to 0. Signed-off-by: Tao Cui <cuitao@kylinos.cn> Reviewed-by: Bibo Mao <maobibo@loongson.cn> Message-ID: <20260626052742.810726-2-cui.tao@linux.dev> Signed-off-by: Song Gao <gaosong@loongson.cn>