tools/virtio: virtio_test: Fix indentation

Replace eight spaces with Tab.
Signed-off-by: Rong Tao <rtoax@foxmail.com>
Message-Id: <tencent_89579C514BC4020324A1A4ACA44B5B95BB07@qq.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>