microvm: wire up hotplug

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

Wire cpu hotplug up for microvm.
Without this we get a broken MADT table.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20200915120909.20838-17-kraxel@redhat.com