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

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: 9040e6dfa8c3fed87695a3de555d2c775727bb51
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190407092314.11066-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>