spapr: add an extra OV5 field to the sPAPR IRQ backend

spapr: add an extra OV5 field to the sPAPR IRQ backend

The interrupt modes supported by the hypervisor are advertised to the
guest with new bits definitions of the option vector 5 of property
"ibm,arch-vec-5-platform-support. The byte 23 bits 0-1 of the OV5 are
defined as follow :

  0b00   PAPR 2.7 and earlier (Legacy systems)
  0b01   XIVE Exploitation mode only
  0b10   Either available

If the client/guest selects the XIVE interrupt mode, it informs the
hypervisor by returning the value 0b01 in byte 23 bits 0-1. A 0b00
value indicates the use of the XICS interrupt mode (Legacy systems).

The sPAPR IRQ backend is extended with these definitions and the
values are directly used to populate the "ibm,arch-vec-5-platform-support"
property. The interrupt mode is advertised under TCG and under KVM.
Although a KVM XIVE device is not yet available, the machine can still
operate with kernel_irqchip=off. However, we apply a restriction on
the CPU which is required to be a POWER9 when a XIVE interrupt
controller is in use.

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