vnc: set id at parse time not init time

This way the generated id will be stored in -writeconfig cfg files.
Also we can make vnc_auto_assign_id() local to vnc.c.

Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qmp.c

@@ -391,7 +391,6 @@ static void qmp_change_vnc_listen(const char *target, Error **errp)
         return;
     }
 
-    vnc_auto_assign_id(olist, opts);
     vnc_display_open("default", errp);
 }