hw/i386/microvm: Fix crash that occurs when introspecting the microvm machine

hw/i386/microvm: Fix crash that occurs when introspecting the microvm machine

QEMU currently crashes when you try to inspect the properties of the
microvm machine:

 $ echo '{ "execute": "qmp_capabilities" }
         { "execute": "qom-list-properties","arguments":
           { "typename": "microvm-machine"}}' | \
   ./qemu-system-x86_64 -qmp stdio
 {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 9},
  "package": "v9.2.0-1072-g60af367187-dirty"}, "capabilities": ["oob"]}}
 {"return": {}}
 qemu-system-x86_64: ../qemu/hw/i386/acpi-microvm.c:250:
  void acpi_setup_microvm(MicrovmMachineState *):
   Assertion `x86ms->fw_cfg' failed.
 Aborted (core dumped)

This happens because the microvm machine adds a machine_done (and a
powerdown_req) notifier in their instance_init function - however, the
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 microvm machine that is never realized. And
since there is already a running 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 microvm_machine_state_init() instead - this
function is the MachineClass->init() function instead, i.e. guaranteed
to be only called once in the lifetime of a QEMU process.

Since the microvm_machine_done() and microvm_powerdown_req() were
defined quite late in the microvm.c file, we have to move them now
also earlier, so that we can get their function pointers from
microvm_machine_state_init() without having to introduce a separate
prototype for those functions earlier.

Reviewed-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250123204708.1560305-1-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>
(cherry picked from commit 38ef383073b8ee59d598643160f206a19a46237f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>