Description of problem:
 Incomplete backing-chain info got by "qemu-img info xxx -U --backing-chain"

Version-Release number of selected component (if applicable):
 kernel version: 4.18.0-57.el8.x86_64
 qemu-kvm version: qemu-kvm-3.1.0-2.module+el8+2606+2c716ad7.x86_64


How reproducible:
 100%

Steps to Reproduce:
1.Start guest with iscsi backend:
   /usr/libexec/qemu-kvm \
    -name 'avocado-vt-vm1' \
    -machine q35  \
    -nodefaults \
    -device VGA,bus=pcie.0,addr=0x1 \
    -device pcie-root-port,id=pcie_root_port_0,slot=2,chassis=2,addr=0x2,bus=pcie.0 \
    -device pcie-root-port,id=pcie_root_port_1,slot=3,chassis=3,addr=0x3,bus=pcie.0 \
    -device pcie-root-port,id=pcie_root_port_2,slot=4,chassis=4,addr=0x4,bus=pcie.0  \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/var/tmp/monitor-qmpmonitor1-20181225-045358-gDqDsfCu,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control  \
    -chardev socket,id=qmp_id_catch_monitor,path=/var/tmp/monitor-catch_monitor-20181225-045358-gDqDsfCu,server,nowait \
    -mon chardev=qmp_id_catch_monitor,mode=control \
    -device pvpanic,ioport=0x505,id=idHtZPbY  \
    -chardev socket,id=serial_id_serial0,path=/var/tmp/serial-serial0-20181225-045358-gDqDsfCu,server,nowait \
    -device isa-serial,chardev=serial_id_serial0  \
    -chardev socket,id=seabioslog_id_20181225-045358-gDqDsfCu,path=/var/tmp/seabios-20181225-045358-gDqDsfCu,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20181225-045358-gDqDsfCu,iobase=0x402 \
    -device pcie-root-port,id=pcie.0-root-port-5,slot=5,chassis=5,addr=0x5,bus=pcie.0 \
    -device qemu-xhci,id=usb1,bus=pcie.0-root-port-5,addr=0x0 \
    -blockdev driver=file,node-name=driveimage1,filename=/mnt/test/rhel80-64-virtio.raw \
    -blockdev node-name=drive_image1,file=driveimage1,driver=raw \
    -device pcie-root-port,id=pcie.0-root-port-6,slot=6,chassis=6,addr=0x6,bus=pcie.0 \
    -device virtio-blk-pci,id=image1,drive=drive_image1,bootindex=0,bus=pcie.0-root-port-6,addr=0x0 \
    -device pcie-root-port,id=pcie.0-root-port-7,slot=7,chassis=7,addr=0x7,bus=pcie.0 \
    -device virtio-net-pci,mac=9a:14:15:16:17:18,id=idQJqHNz,vectors=4,netdev=idXGWoum,bus=pcie.0-root-port-7,addr=0x0  \
    -netdev tap,id=idXGWoum,vhost=on \
    -m 14336  \
    -smp 10,maxcpus=10,cores=5,threads=1,sockets=2  \
    -cpu 'Broadwell',+kvm_pv_unhalt \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1  \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off,strict=off \
    -enable-kvm \
    -monitor stdio \
    -qmp tcp:0:3000,server,nowait \

2. Create snapshot image sn1,sn2,sn3,sn4 in previous
    {'execute':'blockdev-create','arguments':{'options': {'driver':'file','filename':'/root/sn$i','size':21474836480},'job-id':'job1'}}
    {'execute':'blockdev-add','arguments':{'driver':'file','node-name':'drive_sn$i','filename':'/root/sn$i'}}
    {'execute':'blockdev-create','arguments':{'options': {'driver': 'qcow2','file':'drive_sn$i','size':21474836480},'job-id':'job2'}}
    {'execute':'blockdev-add','arguments':{'driver':'qcow2','node-name':'sn$i','file':'drive_sn$i'}}
    {'execute':'job-dismiss','arguments':{'id':'job1'}}
    {'execute':'job-dismiss','arguments':{'id':'job2'}}

3. Create snapshot chain "base->sn1->sn2->sn3->sn4"
   {"execute":"blockdev-snapshot","arguments":   {"node":"drive_image1","overlay":"sn1"}}
   {"execute":"blockdev-snapshot","arguments":{"node":"sn1","overlay":"sn2"}}
   {"execute":"blockdev-snapshot","arguments":{"node":"sn2","overlay":"sn3"}}
   {"execute":"blockdev-snapshot","arguments":{"node":"sn3","overlay":"sn4"}}

4. Check block info 
   4.1 (qemu)info block
         (qemu) info block
    sn4: /root/sn4 (qcow2)
    Attached to:      /machine/peripheral/image1/virtio-backend
    Cache mode:       writeback
    Backing file:     /root/sn3 (chain depth: 4)

   4.2 (host)#qemu-img info /root/sn4 -U --backing-chain
       # qemu-img info sn4 -U --backing-chain
         image: sn4
         file format: qcow2
         virtual size: 20G (21474836480 bytes)
         disk size: 196K
         cluster_size: 65536
         Format specific information:
           compat: 1.1
           lazy refcounts: false
           refcount bits: 16
           corrupt: false

5. Do block commit from sn3 to sn1:
    {'execute': 'block-commit', 'arguments': { 'device': 'sn4', 'base-node':'sn1', 'top-node':'sn3','job-id':'j1'}}

6. Check block info:
   6.1 (qemu)info block
    sn4: /root/sn4 (qcow2)
        Attached to:      /machine/peripheral/image1/virtio-backend
        Cache mode:       writeback
        Backing file:     /root/sn1 (chain depth: 2)

   6.2(qemu)#qemu-img info /root/sn4 -U --backing-chain
image: sn4
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 580K
cluster_size: 65536
backing file: /root/sn1
backing file format: qcow2
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

image: /root/sn1
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 2.0M
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
   
Actual results:
In step4.2, no backing-chains info displayed.

In step6.2, base info not displayed.


Expected results:

In step4.2, all snapshots info should be displayed.

In step6.2, base info should also be displayed.