IOMMU/PCI: consolidate pdev_type() and cache its result for a given device

IOMMU/PCI: consolidate pdev_type() and cache its result for a given device

Add an "unknown" device types as well as one for PCI-to-PCIe bridges
(the latter of which other IOMMU code with or without this patch
doesn't appear to handle properly).

Make sure we don't mistake a device for which we can't access its
config space as a legacy PCI device (after all we in fact don't know
how to deal with such a device, and hence shouldn't try to).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: "Zhang, Xiantao" <xiantao.zhang@intel.com>