pc: fix crash on attempted cpu unplug

when qemu is started with '-no-acpi' CLI option, an attempt
to unplug a CPU using device_del results in null pointer
dereference at:

  #0 object_get_class
  #1 pc_machine_device_unplug_request_cb
  #2 qmp_marshal_device_del

which is caused by pcms->acpi_dev == NULL due to ACPI support
being disabled.

Considering that ACPI support is necessary for unplug to work,
check that it's enabled and fail unplug request gracefully
if no acpi device were found.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/pc.c

@@ -1842,6 +1842,11 @@ static void pc_cpu_unplug_request_cb(HotplugHandler *hotplug_dev,
     X86CPU *cpu = X86_CPU(dev);
     PCMachineState *pcms = PC_MACHINE(hotplug_dev);
 
+    if (!pcms->acpi_dev) {
+        error_setg(&local_err, "CPU hot unplug not supported without ACPI");
+        goto out;
+    }
+
     pc_find_cpu_slot(MACHINE(pcms), cpu->apic_id, &idx);
     assert(idx != -1);
     if (idx == 0) {