vdpa: fix not using CVQ buffer in case of error

vdpa: fix not using CVQ buffer in case of error

Bug introducing when refactoring.  Otherway, the guest never received
the used buffer.

Fixes: be4278b65fc1 ("vdpa: extract vhost_vdpa_net_cvq_add from vhost_vdpa_net_handle_ctrl_avail")
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20230602173451.1917999-1-eperezma@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>