ipmi: fix qemu crash while migrating with ipmi

Qemu crash in the source side while migrating, after starting ipmi service inside vm.

./x86_64-softmmu/qemu-system-x86_64 --enable-kvm -smp 4 -m 4096 \
-drive file=/work/suse/suse11_sp3_64_vt,format=raw,if=none,id=drive-virtio-disk0,cache=none \
-device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0 \
-vnc :99 -monitor vc -device ipmi-bmc-sim,id=bmc0 -device isa-ipmi-kcs,bmc=bmc0,ioport=0xca2

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffec4268700 (LWP 7657)]
__memcpy_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:2757
(gdb) bt
 #0  __memcpy_ssse3_back () at ../sysdeps/x86_64/multiarch/memcpy-ssse3-back.S:2757
 #1  0x00005555559ef775 in memcpy (__len=3, __src=0xc1421c, __dest=<optimized out>)
     at /usr/include/bits/string3.h:51
 #2  qemu_put_buffer (f=0x555557a97690, buf=0xc1421c <Address 0xc1421c out of bounds>, size=3)
     at migration/qemu-file.c:346
 #3  0x00005555559eef66 in vmstate_save_state (f=f@entry=0x555557a97690,
     vmsd=0x555555f8a5a0 <vmstate_ISAIPMIKCSDevice>, opaque=0x555557231160,
     vmdesc=vmdesc@entry=0x55555798cc40) at migration/vmstate.c:333
 #4  0x00005555557cfe45 in vmstate_save (f=f@entry=0x555557a97690, se=se@entry=0x555557231de0,
     vmdesc=vmdesc@entry=0x55555798cc40) at /mnt/sdb/zyy/qemu/migration/savevm.c:720
 #5  0x00005555557d2be7 in qemu_savevm_state_complete_precopy (f=0x555557a97690,
     iterable_only=iterable_only@entry=false) at /mnt/sdb/zyy/qemu/migration/savevm.c:1128
 #6  0x00005555559ea102 in migration_completion (start_time=<synthetic pointer>,
     old_vm_running=<synthetic pointer>, current_active_state=<optimized out>,
     s=0x5555560eaa80 <current_migration.44078>) at migration/migration.c:1707
 #7  migration_thread (opaque=0x5555560eaa80 <current_migration.44078>) at migration/migration.c:1855
 #8  0x00007ffff3900dc5 in start_thread (arg=0x7ffec4268700) at pthread_create.c:308
 #9  0x00007fffefc6c71d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:113

Signed-off-by: Zhuang Yanying <ann.zhuangyanying@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
