vdpa/mlx5: Initialize CVQ vringh only once

Currently, CVQ vringh is initialized inside setup_virtqueues() which is
called every time a memory update is done. This is undesirable since it
resets all the context of the vring, including the available and used
indices.

Move the initialization to mlx5_vdpa_set_status() when
VIRTIO_CONFIG_S_DRIVER_OK is set.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20220613075958.511064-2-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>