vhost: Fix vhostfd leak in error branch

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1417166789-1960-1-git-send-email-arei.gonglei@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
