VNC: Fix memory allocation (wrong structure size).

Pointer vs addresses a VncDisplay structure,
so it is sufficient to allocate sizeof(VncDisplay)
or sizeof(*vs) bytes instead of the much larger
sizeof(VncState).

Maybe the misleading name should be fixed, too:
the code contains many places where vs is used,
sometimes it is a VncState *, sometimes it is a
VncDisplay *. vd would be a better name.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>