ppc/xive: Fix high prio group interrupt being preempted by low prio VP

ppc/xive: Fix high prio group interrupt being preempted by low prio VP

xive_tctx_pipr_present() as implemented with xive_tctx_pipr_update()
causes VP-directed (group==0) interrupt to be presented in PIPR and NSR
despite being a lower priority than the currently presented group
interrupt.

This must not happen. The IPB bit should record the low priority VP
interrupt, but PIPR and NSR must not present the lower 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/20250512031100.439842-32-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>