[libvirt] surprising <backingStore type='file'> setting in domain.xml

[Thomas Stein]

Hello all.

My currently used versions: libvirt-5.2.0 and qemu-4.0.0.

Here is my problem. I'm struggeling since a few weeks with a strange 
behaviour by either qemu or libvirt. After a reboot of
the hardware node the $domain.xml contains suddenly a backingStore 
setting which was not there before reboot.
Something like that:

   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type='file' device='disk'>
       <driver name='qemu' type='qcow2'/>
       <source 
file='/var/lib/libvirt/shinymail/shinymail_weekly.qcow2-2019-05-15'/>
       <backingStore type='file'>
         <format type='qcow2'/>
         <source file='/var/lib/libvirt/images/shinymail.qcow2'/>
       </backingStore>
       <target dev='vda' bus='virtio'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x07' 
function='0x0'/>
     </disk>
...

This obviousely happens after a backup has been running. The Backup 
Script looks like this:

<snip>
virsh snapshot-create-as --domain shinymail weekly --diskspec 
vda,file=/var/lib/libvirt/shinymail/shinymail_weekly.qcow2-$(date 
+%Y-%m-%d) --disk-only --atomic --no-metadata

cp ...

virsh blockcommit shinymail vda --active --verbose --pivot
<snip>

So after that "dmblklist shinymail" does show the right source file but 
after a reboot it tries to use the weekly snapshot
again which leads to filesystem errors.

Someone has an idea what could cause such a behaviour?

cheers
t.