vdpa: export vhost_vdpa_set_vring_ready

vdpa: export vhost_vdpa_set_vring_ready

The vhost-vdpa net backend needs to enable vrings in a different order
than default, so export it.

No functional change intended except for tracing, that now includes the
(virtio) index being enabled and the return value of the ioctl.

Still ignoring return value of this function if called from
vhost_vdpa_dev_start, as reorganize calling code around it is out of
the scope of this series.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230822085330.3978829-3-eperezma@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>