New VirtIO network feature: VIRTIO_NET_F_NOTF_COAL.

Control a Virtio network device notifications coalescing parameters
using the control virtqueue.

A device that supports this fetature can receive
VIRTIO_NET_CTRL_NOTF_COAL control commands.

- VIRTIO_NET_CTRL_NOTF_COAL_TX_SET:
  Ask the network device to change the following parameters:
  - tx_usecs: Maximum number of usecs to delay a TX notification.
  - tx_max_packets: Maximum number of packets to send before a
    TX notification.

- VIRTIO_NET_CTRL_NOTF_COAL_RX_SET:
  Ask the network device to change the following parameters:
  - rx_usecs: Maximum number of usecs to delay a RX notification.
  - rx_max_packets: Maximum number of packets to receive before a
    RX notification.

VirtIO spec. patch:
https://lists.oasis-open.org/archives/virtio-comment/202206/msg00100.htmlSigned-off-by: NAlvaro Karsz <alvaro.karsz@solid-run.com>
Message-Id: <20220718091102.498774-1-alvaro.karsz@solid-run.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NJakub Kicinski <kuba@kernel.org>
Acked-by: NJason Wang <jasowang@redhat.com>