hw/9pfs/virtio-9p-posix-acl: Fix out-of-bounds access

It's detected by coverity. Fix out-of-bounds access of the function mp_dacl_listxattr.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
