virtio-mmio : fix the crash in the vm shutdown

virtio-mmio : fix the crash in the vm shutdown

The root cause for this crash is the ioeventfd not stopped while the VM stop.
The callback for vmstate_change was not implement in virtio-mmio bus

Reproduce step
load the vm with
 -M microvm \
  -netdev tap,id=net0,vhostforce,script=no,downscript=no  \
  -device virtio-net-device,netdev=net0\

After the VM boot, login the vm and then shutdown the vm

System will crash
[Current thread is 1 (Thread 0x7ffff6edde00 (LWP 374378))]
(gdb) bt
0  0x00005555558f18b4 in qemu_flush_or_purge_queued_packets (purge=false, nc=0x55500252e850) at ../net/net.c:636
1  qemu_flush_queued_packets (nc=0x55500252e850) at ../net/net.c:656
2  0x0000555555b6c363 in virtio_queue_notify_vq (vq=0x7fffe7e2b010) at ../hw/virtio/virtio.c:2339
3  virtio_queue_host_notifier_read (n=0x7fffe7e2b08c) at ../hw/virtio/virtio.c:3583
4  0x0000555555de7b5a in aio_dispatch_handler (ctx=ctx@entry=0x5555567c5780, node=0x555556b83fd0) at ../util/aio-posix.c:329
5  0x0000555555de8454 in aio_dispatch_ready_handlers (ready_list=<optimized out>, ctx=<optimized out>) at ../util/aio-posix.c:359
6  aio_poll (ctx=0x5555567c5780, blocking=blocking@entry=false) at ../util/aio-posix.c:662
7  0x0000555555cce0cc in monitor_cleanup () at ../monitor/monitor.c:645
8  0x0000555555b06bd2 in qemu_cleanup () at ../softmmu/runstate.c:822
9  0x000055555586e693 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at ../softmmu/main.c:51

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20211109023744.22387-1-lulu@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>