X86CPU QOM type is in good hands and actively maintained these days, so
drop it from the generic QOM CPU subsystem.

Some refactorings and design questions will still intersect, but review
and discussions of individual series can still take place while opting out
of general X86CPU patch review.
Acked-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>