genirq/msi: Convert storage to xarray

The current linked list storage for MSI descriptors is suboptimal in
several ways:

  1) Looking up a MSI desciptor requires a O(n) list walk in the worst case

  2) The upcoming support of runtime expansion of MSI-X vectors would need
     to do a full list walk to figure out whether a particular index is
     already associated.

  3) Runtime expansion of sparse allocations is even more complex as the
     current implementation assumes an ordered list (increasing MSI index).

Use an xarray which solves all of the above problems nicely.

Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Michael Kelley <[email protected]>
Tested-by: Nishanth Menon <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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