virt: VT-d (PCI passthrough) MSI trap injection

Xen 4.1 before 4.1.1 and 4.0 before 4.0.2, when using PCI passthrough on Intel VT-d chipsets that do not have interrupt remapping, allows guest OS users to gain host OS privileges by "using DMA to generate MSI interrupts by writing to the interrupt injection registers."...