x86: DMs may cause mem leak by IRQ binding

When guests are terminated, various pieces of cleanup need carrying out. The cleaning up of PCI devices which were assigned to guests, and the associated removal of tracking structures for IRQs used by the devices occurs relatively early in the process. Unfortunately after that point the guest about to be terminated could cause its device model (DM) to re-establish such tracking structures, by having it bind one or more IRQs anew. While some of those tracking structures would still be cleaned up later on, at least one would not be.