vhost_test: remove vhost_test_flush_vq()

vhost_test: remove vhost_test_flush_vq()

vhost_test_flush_vq() just a simple wrapper around vhost_work_dev_flush()
which seems have no value. It's just easier to call vhost_work_dev_flush()
directly. Besides there is no point in obtaining vhost_dev pointer
via 'n->vqs[index].poll.dev' while we can just use &n->dev.
It's the same pointers, see vhost_test_open()/vhost_dev_init().

Signed-off-by: Andrey Ryabinin <arbn@yandex-team.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220517180850.198915-5-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>