e500: Replace open-coded loop with qemu_get_cpu()

Since we still need env for ppc-specific fields, obtain it via the new
env_ptr fields to avoid "cpu" name conflicts between CPUState and
PowerPCCPU for now.

This fixes a potential issue with env being NULL at the end of the loop
but cpu still being a valid pointer corresponding to a previous env.

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