vdpa: extract vhost_vdpa_net_cvq_add from vhost_vdpa_net_handle_ctrl_avail

vdpa: extract vhost_vdpa_net_cvq_add from vhost_vdpa_net_handle_ctrl_avail

So we can reuse it to inject state messages.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
--
v7:
* Remove double free error

v6:
* Do not assume in buffer sent to the device is sizeof(virtio_net_ctrl_ack)

v5:
* Do not use an artificial !NULL VirtQueueElement
* Use only out size instead of iovec dev_buffers for these functions.

Signed-off-by: Jason Wang <jasowang@redhat.com>