net/virtio: return early when failover primary alread added

Bail out when primary device was already added before.
This avoids printing a wrong warning message during reboot.

Fixes: 9711cd0d ("net/virtio: add failover support")
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>