vIRQ event channel binding may break Xenstore

Xenstore, to have an up-to-date picture of the entire system, wants to

know of domains appearing and disappearing. To make this more robust, a

new XEN_DOMCTL_get_domain_state was introduced. The management of the

bitmap underlying that operation is tied into the binding of the

VIRQ_DOM_EXC virtual IRQ. Unfortunately an error path there would tear

down the bitmap even in cases when it wasn't set up. Unprivileged domains

can trigger that error path.