Provide cpu hotplug support to Xen. Note this hotplug

Provide cpu hotplug support to Xen. Note this hotplug
support is specific to PM, instead of for a run-time
single CPU hotplug which can be a separate task. See
embedded comment:

/*
 * XXX: One important thing missed here is to migrate vcpus
 * from dead cpu to other online ones and then put whole
 * system into a stop state. It assures a safe environment
 * for a cpu hotplug/remove at normal running state.
 *
 * However for xen PM case, at this point:
 *     -> All other domains should be notified with PM event,
 *        and then in following states:
 *             * Suspend state, or
 *             * Paused state, which is a force step to all
 *               domains if they do nothing to suspend
 *     -> All vcpus of dom0 (except vcpu0) have already beem
 *        hot removed
 * with the net effect that all other cpus only have idle vcpu
 * running. In this special case, we can avoid vcpu migration
 * then and system can be considered in a stop state.
 *
 * So current cpu hotplug is a special version for PM specific
 * usage, and need more effort later for full cpu hotplug.
 * (ktian1)
 */

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Keir Fraser <keir@xensource.com>