spapr: add hotplug interrupt machine options

spapr: add hotplug interrupt machine options

This adds machine options of the form:

  -machine pseries,modern-hotplug-events=true
  -machine pseries,modern-hotplug-events=false

If false, QEMU will force the use of "legacy" style hotplug events,
which are surfaced through EPOW events instead of a dedicated
hot plug event source, and lack certain features necessary, mainly,
for memory unplug support.

If true, QEMU will enable support for "modern" dedicated hot plug
event source. Note that we will still default to "legacy" style unless
the guest advertises support for the "modern" hotplug events via
ibm,client-architecture-support hcall during early boot.

For pseries-2.7 and earlier we default to false, for newer machine
types we default to true.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>