When migrate vm and vm’s disks target host qemu crash due to an invalid free.

#0  object_unref (obj=0x1000) at /qemu-2.12/rpmbuild/BUILD/qemu-2.12/qom/object.c:920
#1  0x0000560434d79e79 in memory_region_unref (mr=<optimized out>)
    at /qemu-2.12/rpmbuild/BUILD/qemu-2.12/memory.c:1730
#2  flatview_destroy (view=0x560439653880) at /qemu-2.12/rpmbuild/BUILD/qemu-2.12/memory.c:292
#3  0x000056043514dfbe in call_rcu_thread (opaque=<optimized out>)
    at /qemu-2.12/rpmbuild/BUILD/qemu-2.12/util/rcu.c:284
#4  0x00007fbc2b36fe25 in start_thread () from /lib64/libpthread.so.0
#5  0x00007fbc2b099bad in clone () from /lib64/libc.so.6

test base qemu-2.12.0，but use lastest qemu(v6.0.0-rc2) also reproduce.
As follow patch can resolve this problem:
https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg02272.html

Steps to reproduce:
(1) Create VM (virsh define)
(2) Add 64 virtio scsi disks
(3) migrate vm and vm’disks
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