Commit 946fb27c moved all the uses of all_cpu_threads_idle()
into cpus.c. This means we can mark the function 'static'
(again), if we shuffle it a bit earlier in the source file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>