cpu: Use QTAILQ for CPU list

Introduce CPU_FOREACH(), CPU_FOREACH_SAFE() and CPU_NEXT() shorthand
macros.

Signed-off-by: Andreas Färber <afaerber@suse.de>
cpu: Drop qemu_for_each_cpu()

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>