spapr_hcall: Replace open-coded CPU loop with qemu_get_cpu()

The helper functions all access ppc-specific fields only so don't bother
to change arguments to PowerPCCPU and use env_ptr instead.

No functional change.

Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
