nbd: Fix overflow return value

The value of reply.error should be the type unsigned int.

Signed-off-by: Yik Fang <eric.fangyi@huawei.com>
Message-Id: <1423722111-12902-1-git-send-email-eric.fangyi@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
