ppc/xive2: add interrupt priority configuration flags

ppc/xive2: add interrupt priority configuration flags

Adds support for extracting additional configuration flags from
the XIVE configuration register that are needed for redistribution
of group interrupts.

Signed-off-by: Glenn Miles <milesg@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Michael Kowal <kowal@linux.ibm.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Link: https://lore.kernel.org/qemu-devel/20250512031100.439842-22-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>