cpu: Replace qemu_for_each_cpu()

It was introduced to loop over CPUs from target-independent code, but
since commit 182735ef target-independent
CPUState is used.

A loop can be considered more efficient than function calls in a loop,
and CPU_FOREACH() hides implementation details just as well, so use that
instead.

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