Since net_init_vhost_vdpa is trying to open it. Not specifying it in the
command line crash qemu.

Fixes: 7327813d ("vhost-vdpa: open device fd in net_init_vhost_vdpa()")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20211112193431.2379298-3-eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>