x86: move cpu hotplug from pc to x86

The cpu hotplug code handles the initialization of coldplugged cpus
too, so it is needed even in case cpu hotplug is not supported.

Move the code from pc to x86, so microvm can use it.

Move both plug and unplug to keep everything in one place, even
though microvm needs plug only.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20200915120909.20838-16-kraxel@redhat.com