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.