xen/acpi: ACPI cpu hotplug

This patch implement real Xen ACPI cpu hotplug driver as module.
When loaded, it replaces Xen stub driver.

For booting existed cpus, the driver enumerates them.
For hotadded cpus, which added at runtime and notify OS via
device or container event, the driver is invoked to add them,
parsing cpu information, hypercalling to Xen hypervisor to add
them, and finally setting up new /sys interface for them.

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>