i386/kvm: fix a use-after-free when vcpu plug/unplug

i386/kvm: fix a use-after-free when vcpu plug/unplug

When we hotplug vcpus, cpu_update_state is added to vm_change_state_head
in kvm_arch_init_vcpu(). But it forgot to delete in kvm_arch_destroy_vcpu() after
unplug. Then it will cause a use-after-free access. This patch delete it in
kvm_arch_destroy_vcpu() to fix that.

Reproducer:
    virsh setvcpus vm1 4 --live
    virsh setvcpus vm1 2 --live
    virsh suspend vm1
    virsh resume vm1

The UAF stack:
==qemu-system-x86_64==28233==ERROR: AddressSanitizer: heap-use-after-free on address 0x62e00002e798 at pc 0x5573c6917d9e bp 0x7fff07139e50 sp 0x7fff07139e40
WRITE of size 1 at 0x62e00002e798 thread T0
    #0 0x5573c6917d9d in cpu_update_state /mnt/sdb/qemu/target/i386/kvm.c:742
    #1 0x5573c699121a in vm_state_notify /mnt/sdb/qemu/vl.c:1290
    #2 0x5573c636287e in vm_prepare_start /mnt/sdb/qemu/cpus.c:2144
    #3 0x5573c6362927 in vm_start /mnt/sdb/qemu/cpus.c:2150
    #4 0x5573c71e8304 in qmp_cont /mnt/sdb/qemu/monitor/qmp-cmds.c:173
    #5 0x5573c727cb1e in qmp_marshal_cont qapi/qapi-commands-misc.c:835
    #6 0x5573c7694c7a in do_qmp_dispatch /mnt/sdb/qemu/qapi/qmp-dispatch.c:132
    #7 0x5573c7694c7a in qmp_dispatch /mnt/sdb/qemu/qapi/qmp-dispatch.c:175
    #8 0x5573c71d9110 in monitor_qmp_dispatch /mnt/sdb/qemu/monitor/qmp.c:145
    #9 0x5573c71dad4f in monitor_qmp_bh_dispatcher /mnt/sdb/qemu/monitor/qmp.c:234

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200513132630.13412-1-pannengyuan@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>