When /var/log partition is full, network stack does not start on boot.
This is a big problem because eventually you can loose control over host. With host when you have KVM console, there is no problem. But cheap servers do not.
So, only way to recover is accessing physically to server and delete some logs.
Although I agree that xenserver should not completely initialize if /var/log partition is full, it should trigger at least network stack in order to allow remotely connect for free some space.