virtio-vsock was introduced after the release of VIRTIO 1.0
specifications, so it should be 'modern-only'.

This patch forces virtio version 1 as done for vhost-vsock-pci.

To avoid migration issues, we force virtio version 1 only when
legacy check is enabled in the new machine types (>= 5.1).

Cc: qemu-stable@nongnu.org
Suggested-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200921122506.82515-5-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>