spapr: introduce the XIVE_EXPLOIT option in CAS

On POWER9, the Client Architecture Support (CAS) negotiation process
determines whether the guest operates in XIVE Legacy compatibility
(the former POWER8 interrupt model) or in XIVE exploitation mode (the
newer POWER9 interrupt model).

Bit 7 of Byte 23 of vector 5 is used for this purpose.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
