Suppress ErrorKindNoKernelIOMMU errors on systems with no external facing PCI ports

Some systems are triggering the error about missing kernel DMA remapping, despite the CPU generation being new enough and the opt-in flag being present in the `DMAR` ACPI table. Further debugging would be required to determine why this isn't being enabled, but on the systems we've looked at so far, none of them have any Thunderbolt ports which is what this feature is really required for.

External PCI ports have the `ExternalFacingPort` property in ACPI. We don't really want to be parsing ACPI tables for this, but the kernel does mark PCI bridges with external ports (ie, Thunderbolt) with this property by adding the `removable` attribute in sysfs (with the value `removable`).

We should only surface this error if:
- There are PCI bridges with external facing ports, and
- Any of these bridges are not assigned to an IOMMU.