HOTPLUG: Add CPU_DYING notifier

KVM wants a notification when a cpu is about to die, so it can disable
hardware extensions, but at a time when user processes cannot be scheduled
on the cpu, so it doesn't try to use virtualization extensions after they
have been disabled.

This adds a CPU_DYING notification.  The notification is called in atomic
context on the doomed cpu.

Signed-off-by: Avi Kivity <avi@qumranet.com>