Support including additional GPU functions on different IOMMU groups

[masnax]

Incus' current behaviour with the gpu device type is to auto-load all additional GPU functions in the same IOMMU group.

If GPU functions are on different IOMMU groups, then they are not imported, and adding them as explicit pci devices causes Incus to import them as discrete PCI devices on separate root ports, which then has to be amended with raw.qemu.conf or raw.qemu.scriptlet.

Here, I've added a functions config key that accepts comma delimited additional function numbers (1-7) to enforce inclusion as long as the following path exists on the main device:

ls /sys/bus/pci/devices/0000:40:00.0/ -l | grep consumer
lrwxrwxrwx 1 root root         0 Jun 10 16:57 consumer:pci:0000:40:00.1 -> ../../../../../virtual/devlink/pci:0000:40:00.0--pci:0000:40:00.1
lrwxrwxrwx 1 root root         0 Jun 10 16:57 consumer:pci:0000:40:00.2 -> ../../../../../virtual/devlink/pci:0000:40:00.0--pci:0000:40:00.2
lrwxrwxrwx 1 root root         0 Jun 10 16:57 consumer:pci:0000:40:00.3 -> ../../../../../virtual/devlink/pci:0000:40:00.0--pci:0000:40:00.3

# But not present in the same IOMMU group:
ls /sys/bus/pci/devices/0000:40:00.0/iommu_group/devices  -l
total 0
lrwxrwxrwx 1 root root 0 Jun 10 17:52 0000:40:00.0 -> ../../../../devices/pci0000:00/0000:00:03.2/0000:3e:00.0/0000:3f:00.0/0000:40:00.0

So to include all of these, the devices section would look like this:

devices:
  gpu:
    functions: "1,2,3"
    pci: 40:00.0
    type: gpu

With the changes here, additional functions are enforced as a fallback to the existing IOMMU group inclusions.

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Some additional steps are required before this can be considered ready:

• Properly handle "transactional" unplug of the entire slot. This is related to Allow (un)plugging whole IOMMU groups at once #3422 but with the caveat that IOMMU groups may not be the whole source of truth.
• Potentially use a more generic solution rather than the functions key, so it can apply to non-GPU devices too. This could look like explicit pci and gpu devices, where Incus internally determines if they ought to be treated as the same multifunction device.