ppc/xive2: Support redistribution of group interrupts

ppc/xive2: Support redistribution of group interrupts

When an XIVE context is pulled while it has an active, unacknowledged
group interrupt, XIVE will check to see if a context on another thread
can handle the interrupt and, if so, notify that context.  If there
are no contexts that can handle the interrupt, then the interrupt is
added to a backlog and XIVE will attempt to escalate the interrupt,
if configured to do so, allowing the higher privileged handler to
activate a context that can handle the original interrupt.

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/20250512031100.439842-23-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>