vfio: selftests: Move IOMMU library code into iommu.c

vfio: selftests: Move IOMMU library code into iommu.c

Move all the IOMMU related library code into their own file iommu.c.
This provides a better separation between the vfio_pci_device helper
code and the iommu code.

No function change intended.

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/20251126231733.3302983-12-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>