cpus: Change cpu_thread_is_idle() argument to CPUState

It no longer needs CPUArchState.

Prepares for changing all_cpu_threads_idle() CPU loop to CPUState and
needed for changing qemu_kvm_wait_io_event() argument to CPUState.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>
