Description of problem:
Both qemu and guest hang when performing live snapshot transaction with data-plane. 
It only occurs when use same one iothread object for multiple block. 
If use separate iothreads for different block, the issue does not exist.


Version-Release number of selected component (if applicable):
qemu-kvm-rhev-2.10.0-9.el7.x86_64
kernel-3.10.0-800.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Launch guest with 3 block devices, all start with a same iothread "iothread1":

#  /usr/libexec/qemu-kvm \
     -name 'avocado-vt-vm1'  \
     -object iothread,id=iothread1 \
     -drive id=drive_image1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/rhel74-64-virtio.qcow2 \
     -device virtio-blk-pci,id=image1,drive=drive_image1,bootindex=0,iothread=iothread1,bus=pci.0,addr=0x3 \
     -drive id=drive_sn1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/sn1.qcow2 \
     -device virtio-blk-pci,id=sn1,drive=drive_sn1,bootindex=1,iothread=iothread1,bus=pci.0,addr=0x4 \
     -drive id=drive_sn2,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/sn2.qcow2 \
     -device virtio-blk-pci,id=sn2,drive=drive_sn2,bootindex=2,iothread=iothread1,bus=pci.0,addr=0x5 \
     -device virtio-net-pci,mac=9a:29:2a:2b:2c:2d,id=idoE4ENA,vectors=4,netdev=idgbsOIs,bus=pci.0,addr=0x6  \
     -netdev tap,id=idgbsOIs \
     -m 2048  \
     -smp 4,maxcpus=4,cores=2,threads=1,sockets=2  \
     -cpu 'SandyBridge',+kvm_pv_unhalt \
     -vnc :0  \
     -qmp stdio

2. Live snapshot three block devices together within a transaction command:

{'execute': 'transaction', 'arguments': {'actions': [{'data': {'device':'drive_image1', 'snapshot-file': '/home/kvm_autotest_root/images/image1-snap', 'mode': 'absolute-paths', 'format': 'qcow2'}, 'type': 'blockdev-snapshot-sync'}, {'data': {'device': 'drive_sn1', 'snapshot-file': '/home/kvm_autotest_root/images/sn1-snap', 'mode': 'absolute-paths', 'format': 'qcow2'}, 'type': 'blockdev-snapshot-sync'}, {'data': {'device': 'drive_sn2', 'snapshot-file': '/home/kvm_autotest_root/images/sn2-snap', 'mode': 'absolute-paths', 'format': 'qcow2'}, 'type': 'blockdev-snapshot-sync'}]}, 'id': 'bYyCHqv5'}


Actual results:
Both qemu and guest hang there, qmp only shows formating the first snapshot files, and then hang there without any other returns:

Formatting '/home/kvm_autotest_root/images/image1-snap', fmt=qcow2 size=21474836480 backing_file=/home/kvm_autotest_root/images/rhel74-64-virtio.qcow2 backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16

Expected results:

Formatting '/home/kvm_autotest_root/images/image1-snap', fmt=qcow2 size=21474836480 backing_file=/home/kvm_autotest_root/images/rhel74-64-virtio.qcow2 backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
Formatting '/home/kvm_autotest_root/images/sn1-snap', fmt=qcow2 size=21474836480 backing_file=/home/kvm_autotest_root/images/sn1.qcow2 backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
Formatting '/home/kvm_autotest_root/images/sn2-snap', fmt=qcow2 size=21474836480 backing_file=/home/kvm_autotest_root/images/sn2.qcow2 backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
{"return": {}, "id": "bYyCHqv5"}


Additional info:
If use three different iothreads object for each block as below, both qemu and guest works well, and qmp returned information is same as above expected results.

/usr/libexec/qemu-kvm \
     -name 'avocado-vt-vm1'  \
     -object iothread,id=iothread1 \
     -object iothread,id=iothread2 \
     -object iothread,id=iothread3 \
     -drive id=drive_image1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/rhel74-64-virtio.qcow2 \
     -device virtio-blk-pci,id=image1,drive=drive_image1,bootindex=0,iothread=iothread1,bus=pci.0,addr=0x3 \
     -drive id=drive_sn1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/sn1.qcow2 \
     -device virtio-blk-pci,id=sn1,drive=drive_sn1,bootindex=1,iothread=iothread2,bus=pci.0,addr=0x4 \
     -drive id=drive_sn2,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/sn2.qcow2 \
     -device virtio-blk-pci,id=sn2,drive=drive_sn2,bootindex=2,iothread=iothread3,bus=pci.0,addr=0x5 \
     -device virtio-net-pci,mac=9a:29:2a:2b:2c:2d,id=idoE4ENA,vectors=4,netdev=idgbsOIs,bus=pci.0,addr=0x6  \
     -netdev tap,id=idgbsOIs \
     -m 2048  \
     -smp 4,maxcpus=4,cores=2,threads=1,sockets=2  \
     -cpu 'SandyBridge',+kvm_pv_unhalt \
     -vnc :0  \
     -qmp stdio