hw/i386/pc: Fix crash that occurs when introspecting TYPE_PC_MACHINE machines

hw/i386/pc: Fix crash that occurs when introspecting TYPE_PC_MACHINE machines

QEMU currently crashes when you try to inspect the machines based on
TYPE_PC_MACHINE for their properties:

 $ echo '{ "execute": "qmp_capabilities" }
         { "execute": "qom-list-properties","arguments":
                      { "typename": "pc-q35-10.0-machine"}}' \
   | ./qemu-system-x86_64 -M pc -qmp stdio
 {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 9},
  "package": "v9.2.0-1070-g87e115c122-dirty"}, "capabilities": ["oob"]}}
 {"return": {}}
 Segmentation fault (core dumped)

This happens because TYPE_PC_MACHINE machines add a machine_init-
done_notifier in their instance_init function - but instance_init
of machines are not only called for machines that are realized,
but also for machines that are introspected, so in this case the
listener is added for a q35 machine that is never realized. But
since there is already a running pc machine, the listener function
is triggered immediately, causing a crash since it was not for the
right machine it was meant for.

Such listener functions must never be installed from an instance_init
function. Let's do it from pc_basic_device_init() instead - this
function is called from the MachineClass->init() function instead,
i.e. guaranteed to be only called once in the lifetime of a QEMU
process.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2779
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250117192106.471029-1-thuth@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>