Comment 1 for bug 352302

It sounds like the xenstored was corrupted. The code that's responsible for figuring out which VNC server to connect to is at:

https://xvm.scripts.mit.edu/browser/trunk/packages/invirt-vnc-server/python/vnc/get_port.py

As you can see, it uses the XenAPI to request the information for the specific VM name it is given and then extracts the virtual framebuffer location from that information.