IOMMU/ATS: use a struct pci_dev * instead of SBDF

IOMMU/ATS: use a struct pci_dev * instead of SBDF

Do away with struct pci_ats_dev; integrate the few bits of information
in struct pci_dev (and as a result drop get_ats_device() altogether).
Hook ATS devices onto a linked list off of each IOMMU instead of on a
global one.

Signed-off-by: Quan Xu <quan.xu@intel.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Quan Xu <quan.xu@intel.com>