ppc/xive2: implement NVP context save restore for POOL ring

ppc/xive2: implement NVP context save restore for POOL ring

In preparation to implement POOL context push, add support for POOL
NVP context save/restore.

The NVP p bit is defined in the spec as follows:

    If TRUE, the CPPR of a Pool VP in the NVP is updated during store of
    the context with the CPPR of the Hard context it was running under.

It's not clear whether non-pool VPs always or never get CPPR updated.
Before this patch, OS contexts always save CPPR, so we will assume that
is the behaviour.

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/20250512031100.439842-41-npiggin@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>