net/virtio: add check for cvq

net/virtio: add check for cvq

Add checking for cvq to judge if virtio_ack_link_announce should be called.
The existing code doesn't cause issue, and add the checking just to look
more reasonable.

Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>