virtio-net: failover: add missing remove_migration_state_change_notifier()

virtio-net: failover: add missing remove_migration_state_change_notifier()

In the failover case configuration, virtio_net_device_realize() uses an
add_migration_state_change_notifier() to add a state notifier, but this
notifier is not removed by the unrealize function when the virtio-net
card is unplugged.

If the card is unplugged and a migration is started, the notifier is
called and as it is not valid anymore QEMU crashes.

This patch fixes the problem by adding the
remove_migration_state_change_notifier() in virtio_net_device_unrealize().

The problem can be reproduced with:

  $ qemu-system-x86_64 -enable-kvm -m 1g -M q35 \
    -device pcie-root-port,slot=4,id=root1 \
    -device pcie-root-port,slot=5,id=root2 \
    -device virtio-net-pci,id=net1,mac=52:54:00:6f:55:cc,failover=on,bus=root1 \
    -monitor stdio disk.qcow2
  (qemu) device_del net1
  (qemu) migrate "exec:gzip -c > STATEFILE.gz"

  Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
  0x0000000000000000 in ?? ()
  (gdb) bt
  #0  0x0000000000000000 in  ()
  #1  0x0000555555d726d7 in notifier_list_notify (...)
      at .../util/notify.c:39
  #2  0x0000555555842c1a in migrate_fd_connect (...)
      at .../migration/migration.c:3975
  #3  0x0000555555950f7d in migration_channel_connect (...)
      error@entry=0x0) at .../migration/channel.c:107
  #4  0x0000555555910922 in exec_start_outgoing_migration (...)
      at .../migration/exec.c:42

Reported-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>