hw/i386/pc: Fix crash when hot-plugging nvdimm on older machine types

QEMU currently crashes when you try to hot-plug an "nvdimm" device
on older machine types:

$ qemu-system-x86_64 -monitor stdio -M pc-1.1
QEMU 3.1.92 monitor - type 'help' for more information
(qemu) device_add nvdimm,id=nvdimmn1
qemu-system-x86_64: /home/thuth/devel/qemu/util/error.c:57: error_setv:
 Assertion `*errp == ((void *)0)' failed.
Aborted (core dumped)

The call to hotplug_handler_pre_plug() in pc_memory_pre_plug() has been
added recently before the check whether nvdimm is enabled. It should
be done after the check. And while we're at it, also check the errp
after the hotplug_handler_pre_plug(), otherwise errors are silently
ignored here.

Fixes: 9040e6d
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190407092314.11066-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/pc.c

@@ -2078,6 +2078,7 @@ static void pc_memory_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
     const MachineState *ms = MACHINE(hotplug_dev);
     const bool is_nvdimm = object_dynamic_cast(OBJECT(dev), TYPE_NVDIMM);
     const uint64_t legacy_align = TARGET_PAGE_SIZE;
+    Error *local_err = NULL;
 
     /*
      * When -no-acpi is used with Q35 machine type, no ACPI is built,
@@ -2090,13 +2091,17 @@ static void pc_memory_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
         return;
     }
 
-    hotplug_handler_pre_plug(pcms->acpi_dev, dev, errp);
-
     if (is_nvdimm && !ms->nvdimms_state->is_enabled) {
         error_setg(errp, "nvdimm is not enabled: missing 'nvdimm' in '-M'");
         return;
     }
 
+    hotplug_handler_pre_plug(pcms->acpi_dev, dev, &local_err);
+    if (local_err) {
+        error_propagate(errp, local_err);
+        return;
+    }
+
     pc_dimm_pre_plug(PC_DIMM(dev), MACHINE(hotplug_dev),
                      pcmc->enforce_aligned_dimm ? NULL : &legacy_align, errp);
 }