QMP: Introduce VNC_INITIALIZED event

It's emitted when a VNC client session is activated by QEMU,
client's information such as port, IP and auth ID (if the
session is authenticated) are provided.

Event example:

{ "event": "VNC_INITIALIZED",
    "timestamp": {"seconds": 1263475302, "microseconds": 150772},
    "data": {
        "server": { "auth": "sasl", "family": "ipv4",
                    "service": "5901", "host": "0.0.0.0"},
        "client": { "family": "ipv4", "service": "46089",
                    "host": "127.0.0.1", "sasl_username": "lcapitulino" } } }

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