virtio-net: check feature before configuring the vq coalescing command

Virtio spec says:

	The driver MUST have negotiated the VIRTIO_NET_F_VQ_NOTF_COAL
	feature when issuing commands VIRTIO_NET_CTRL_NOTF_COAL_VQ_SET
	and VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET.

So we add the feature negotiation check to
virtnet_send_{r,t}x_ctrl_coal_vq_cmd as a basis for the next bugfix patch.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>