vhost-vdpa: negotiate VIRTIO_NET_F_STATUS with driver

Vendor driver may not support or implement config
interrupt delivery for link status notifications.
In this event, vendor driver is expected to NACK
the feature, but guest will keep link always up.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Message-Id: <1601582985-14944-1-git-send-email-si-wei.liu@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>