IRQ vector leak on x86

ISSUE DESCRIPTION

An x86 HVM guest with PCI pass through devices can force the allocation of all IDT vectors on the system by rebooting itself with MSI or MSI-X capabilities enabled and entries setup.
Such reboots will leak any vectors used by the MSI(-X) entries that the guest might had enabled, and hence will lead to vector exhaustion on the system, not allowing further PCI pass through devices to work properly.

IMPACT

HVM guests with PCI pass through devices can mount a Denial of Service (DoS) attack affecting the pass through of PCI devices to other guests or the hardware domain. In the latter case this would affect the entire host.

VULNERABLE SYSTEMS

Xen versions 4.12.3, 4.12.4, and all versions from 4.13.1 onwards are vulnerable. Xen version 4.13.0 and all versions up to 4.12.2 are not affected.
Only x86 systems running HVM guests with PCI pass through devices are vulnerable.