target-ppc: Introduce unrealizefn for PowerPCCPU

Use it to clean up the opcode table, resolving a former TODO from Jocelyn.
Also switch from malloc() to g_malloc().

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

Signed-off-by: Andreas Färber <afaerber@suse.de>
monitor: Use qemu_get_cpu() in monitor_set_cpu()

No functional change, just a reduction of CPU loops.

The mon_cpu field is left untouched for now since changing that requires
a number of larger prerequisites, including cpu_synchronize_state() and
mon_get_cpu().

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