pci: phantom functions assigned to incorrect contexts

PCI devices can make use of a functionality called phantom functions,

that when enabled allows the device to generate requests using the IDs

of functions that are otherwise unpopulated. This allows a device to

extend the number of outstanding requests.

Such phantom functions need an IOMMU context setup, but failure to

setup the context is not fatal when the device is assigned. Not

failing device assignment when such failure happens can lead to the

primary device being assigned to a guest, while some of the phantom

functions are assigned to a different domain.