QMP: Introduce VNC_CONNECTED event

It's emitted when a VNC client connects to QEMU, client's information
such as port and IP address are provided.

Note that this event is emitted right when the connection is
established. This means that it happens before authentication
procedure and session initialization.

Event example:

{ "event": "VNC_CONNECTED",
    "timestamp": { "seconds": 1262976601, "microseconds": 975795 },
    "data": {
        "server": { "auth": "sasl", "family": "ipv4",
                    "service": "5901", "host": "0.0.0.0" },
        "client": { "family": "ipv4", "service": "58425",
                    "host": "127.0.0.1" } } }

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
