MSI fixes for 6.2:

- Return MSI_XA_DOMAIN_SIZE as the maximum MSI index when the architecture
  does not make use of irq domains instead of returning 0, which is pretty
  limiting.

- Check for the presence of an irq domain when validating the MSI iterator,
  as s390/powerpc won't have one.

- Fix powerpc's MSI backends which fail to clear the descriptor's IRQ field
  on teardown, leading to a splat and leaked descriptors.
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