spapr_rtas: Abstract rtas_query_cpu_stopped_state() with qemu_get_cpu()

Instead of looping over all CPUArchState, use a helper to obtain the
desired CPUState directly. Saves a CPUPPCState variable and QOM cast.

Signed-off-by: Andreas Färber <afaerber@suse.de>
spapr_rtas: Abstract rtas_start_cpu() with qemu_get_cpu()

Instead of looping over all CPUArchState, use a helper to obtain the
desired CPUState.
Free the "cpu" variable for PowerPCCPU, to access its CPUPPCState.

Signed-off-by: Andreas Färber <afaerber@suse.de>