ppc/xive2: Use fair irq target search algorithm

The current xive algorithm for finding a matching group vCPU
target always uses the first vCPU found.  And, since it always
starts the search with thread 0 of a core, thread 0 is almost
always used to handle group interrupts.  This can lead to additional
interrupt latency and poor performance for interrupt intensive
work loads.

Changing this to use a simple round-robin algorithm for deciding which
thread number to use when starting a search, which leads to a more
distributed use of threads for handling group interrupts.

[npiggin: Also round-robin among threads, not just cores]

Signed-off-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Caleb Schlossin <calebs@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-9-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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