VNC: Cache client info at connection time

When a disconnection happens the client's socket on QEMU
side may become invalid, this way it won't be possible
to query it to get client information, which is going to
be needed by the future QMP VNC_DISCONNECTED event.

To always have this information available we query the
socket at connection time and cache the client info in
struct VncState.

Two function are introduced to perform this job.

vnc_client_cache_addr() is called right when the connection
is made, however the authentication information is not
available at that moment so vnc_client_cache_auth() is
called from protocol_client_init() to get auth info.

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