tools/virtio: enable to build with retpoline

Add build options to bring it close to a linux kernel. It allows for
testing that is close to reality.

Signed-off-by: Shunsuke Mie <mie@igel.co.jp>
Message-Id: <20230202104538.2041879-1-mie@igel.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>