ppc/xive: Assert group interrupts were redistributed

ppc/xive: Assert group interrupts were redistributed

Add some assertions to try to ensure presented group interrupts do
not get lost without being redistributed, if they become precluded
by CPPR or preempted by a higher priority interrupt.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-40-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>