cpus: Replace open-coded CPU loop in qmp_memsave() with qemu_get_cpu()

No functional change, just less usages of first_cpu and next_cpu fields.

env is passed to cpu_memory_rw_debug(), which in turn passes it to
target-specific cpu_get_phys_page_debug(). Changing both would be a
larger refactoring, so defer that by using env_ptr for now.

Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
