vfio: selftests: Support multiple devices in the same container/iommufd

vfio: selftests: Support multiple devices in the same container/iommufd

Support tests that want to add multiple devices to the same
container/iommufd by decoupling struct vfio_pci_device from
struct iommu.

Multi-devices tests can now put multiple devices in the same
container/iommufd like so:

  iommu = iommu_init(iommu_mode);

  device1 = vfio_pci_device_init(bdf1, iommu);
  device2 = vfio_pci_device_init(bdf2, iommu);
  device3 = vfio_pci_device_init(bdf3, iommu);

  ...

  vfio_pci_device_cleanup(device3);
  vfio_pci_device_cleanup(device2);
  vfio_pci_device_cleanup(device1);

  iommu_cleanup(iommu);

To account for the new separation of vfio_pci_device and iommu, update
existing tests to initialize and cleanup a struct iommu.

Reviewed-by: Alex Mastro <amastro@fb.com>
Tested-by: Alex Mastro <amastro@fb.com>
Reviewed-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20251126231733.3302983-7-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>