From: Rusty Russell <rusty@rustcorp.com.au>

The cpu hotplug code actually provides two notifiers: CPU_UP_PREPARE
which preceeds the online and can fail, and CPU_ONLINE which can't.

Current usage is only done at boot, so this distinction doesn't
matter, but it's a bad example to set.  This also means that the
migration threads do not have to be higher priority than the
others, since they are ready to go before any CPU_ONLINE callbacks
are done.

This patch is experimental but fairly straight foward: I haven't been
able to test it since extracting it from the hotplug cpu code, so it's
possible I screwed something up.