spapr: Memory hot-unplug support

spapr: Memory hot-unplug support

Add support to hot remove pc-dimm memory devices.

Since we're introducing a machine-level unplug_request hook, we also
had handling for CPU unplug there as well to ensure CPU unplug
continues to work as it did before.

Signed-off-by: Bharata B Rao <[email protected]>
* add hooks to CAS/cmdline enablement of hotplug ACR support
* add hook for CPU unplug
Signed-off-by: Michael Roth <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Signed-off-by: David Gibson <[email protected]>