x86: boot cpus from cpu_up, instead of prepare_cpus

After all the infrastructure work, we're now prepared
to boot the cpus from cpu_up, and not from prepare_cpus.
So the difference between cold boot and hotplug is effectively
over, and the functions are used to the purposes they're meant to.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>