QMP: Introduce VNC_DISCONNECTED event

QMP: Introduce VNC_DISCONNECTED event

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

Event example:

{ "event": "VNC_DISCONNECTED",
    "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", "sasl_username": "foo" } } }

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