libvduse: Pass positive value to strerror()

The value passed to strerror() should be positive.
So let's fix it.

Fixes: Coverity CID 1490226, 1490223
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20220706095624.328-4-xieyongji@bytedance.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>