spapr: Unrealize vCPUs with qdev_unrealize()

Since we introduced CPU hot-unplug in sPAPR, we don't unrealize the
vCPU objects explicitly. Instead, we let QOM handle that for us under
object_property_del_all() when the CPU core object is finalized. The
only thing we do is calling cpu_remove_sync() to tear the vCPU thread
down.

This happens to work but it is ugly because:
- we call qdev_realize() but the corresponding qdev_unrealize() is
  buried deep in the QOM code
- we call cpu_remove_sync() to undo qemu_init_vcpu() called by
  ppc_cpu_realize() in target/ppc/translate_init.c.inc
- the CPU init and teardown paths aren't really symmetrical

The latter didn't bite us so far but a future patch that greatly
simplifies the CPU core realize path needs it to avoid a crash
in QOM.

For all these reasons, have ppc_cpu_unrealize() to undo the changes
of ppc_cpu_realize() by calling cpu_remove_sync() at the right place,
and have the sPAPR CPU core code to call qdev_unrealize().

This requires to add a missing stub because translate_init.c.inc is
also compiled for user mode.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160279671236.1808373.14732005038172874990.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target/ppc/translate_init.c.inc

@@ -10328,6 +10328,8 @@ static void ppc_cpu_unrealize(DeviceState *dev)
 
     pcc->parent_unrealize(dev);
 
+    cpu_remove_sync(CPU(cpu));
+
     for (i = 0; i < PPC_CPU_OPCODES_LEN; i++) {
         if (cpu->opcodes[i] == &invalid_handler) {
             continue;