net: stream: add QAPI events to report connection state

net: stream: add QAPI events to report connection state

The netdev reports NETDEV_STREAM_CONNECTED event when the backend
is connected, and NETDEV_STREAM_DISCONNECTED when it is disconnected.

The NETDEV_STREAM_CONNECTED event includes the destination address.

This allows a system manager like libvirt to detect when the server
fails.

For instance with passt:

{ 'execute': 'qmp_capabilities' }
{ "return": { } }
{ "timestamp": { "seconds": 1666341395, "microseconds": 505347 },
    "event": "NETDEV_STREAM_CONNECTED",
    "data": { "netdev-id": "netdev0",
        "addr": { "path": "/tmp/passt_1.socket", "type": "unix" } } }

[killing passt here]

{ "timestamp": { "seconds": 1666341430, "microseconds": 968694 },
    "event": "NETDEV_STREAM_DISCONNECTED",
    "data": { "netdev-id": "netdev0" } }

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>