Description of problem:
Refresh pool failed when storage pool contains volume with broken backing store

Version-Release number of selected component (if applicable):
libvirt-7.0.0-8.module+el8.4.0+10233+8b7fd9eb.x86_64
qemu-kvm-5.2.0-10.module+el8.4.0+10217+cbdd2152.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Prepare a dir pool.
# virsh pool-dumpxml default 
<pool type='dir'>
  <name>default</name>
  <uuid>7932d41a-daec-4522-b405-d3f0e92b3092</uuid>
  <capacity unit='bytes'>75125227520</capacity>
  <allocation unit='bytes'>17200705536</allocation>
  <available unit='bytes'>57924521984</available>
  <source>
  </source>
  <target>
    <path>/var/lib/libvirt/images</path>
    <permissions>
      <mode>0711</mode>
      <owner>0</owner>
      <group>0</group>
      <label>system_u:object_r:virt_image_t:s0</label>
    </permissions>
  </target>
</pool>

2. Create an image with a broken backing store in the storage pool path.
# qemu-img create -f qcow2 -F qcow2 -b 'json:{' -u /var/lib/libvirt/images/test.img 200M
Formatting '/var/lib/libvirt/images/test.img', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=209715200 backing_file=json:{ backing_fmt=qcow2 lazy_refcounts=off refcount_bits=16

3. Refresh the default pool.
# virsh pool-refresh default 
error: Failed to refresh pool default
error: internal error: cannot parse json {: parse error: premature EOF
                                       {
                     (right here) ------^


Actual results:
Refresh the pool failed

Expected results:
Refresh the pool successfully

Additional info:
According to https://bugzilla.redhat.com/show_bug.cgi?id=1933050#c8, use this new bug to track the upstream fixes:
commit 104db1951d3abfd8cb363b8e8070f712044bc645
Author: Peter Krempa <pkrempa>
Date:   Thu Feb 25 13:51:51 2021 +0100

    storageBackendProbeTarget: Don't fail if backing store can't be parsed
    
    When the backing store of the image can't be parsed
    virStorageSourceNewFromBacking returns -1. storageBackendProbeTarget
    then also fails which makes the pool refresh fail or even the storage
    pool becomes inactive after (re)start of libvirtd.
    
    In situations when we can't access the backing store via network we
    just report the backing store string, thus we can do the same thing for
    unparsable backing store to prevent the pool from going offline.