libvirt users - Jun 2016 - Problem executing VM backups

Hi everyone,

we are suddenly having a problem with executing our backup jobs. For a 
long time, we have used a shell script which contains the following code 
to backup all our virtual machines:

         for domain in Testserver Faktura Fileserver Gitolite Jenkins 
Nexus SimpleHelp VpnGateway Wiki; do
                 echo -n "$(date +"%Y-%m-%d %H:%M:%S") starting
backup
for vm ${domain} ... " >> ${vmlog}
                 virsh dumpxml --security-info ${domain} > 
${vmdir}/${domain}.xml
                 virsh undefine ${domain}  >> ${vmlog}
                 virsh blockcopy ${domain} 
/var/lib/libvirt/images/${domain}.img ${vmdir}/${domain}.img --wait 
--finish  >> ${vmlog}
                 virsh define ${vmdir}/${domain}.xml >> ${vmlog}
         done

This has worked great for us, but all of the sudden (eventually 
triggered by an update, since of course we do regular security/package 
updates on this machine) we are having problems. For some virtual 
machines, it still works perfectly, but for others, virsh tells us that 
a blockjob is still active and therefore the backup fails. This seems to 
happen to machines at random. However, when we then try to query the 
active blockjob, virsh tells us that no blockjob is active. Consider the 
following log from the shell:

                  root@gfii-host:~# virsh undefine Gitolite
                  error: Failed to undefine domain Gitolite
                  error: Requested operation is not valid: cannot 
undefine transient domain

                  root@gfii-host:~# virsh blockcopy Gitolite 
/var/lib/libvirt/images/Gitolite.img /tmp/test-blockcopy-gitolite.img 
--wait --finish
                  error: block copy still active: disk 'vda' already in 
active block job

                  root@gfii-host:~# virsh blockjob Gitolite 
/var/lib/libvirt/images/Gitolite.img
                  No current block job for 
/var/lib/libvirt/images/Gitolite.img
                  root@gfii-host:~# virsh define 
/var/local/backup/vms/2016-06-22T013001/Gitolite.xml
                  error: Failed to define domain from 
/var/local/backup/vms/2016-06-22T013001/Gitolite.xml
                  error: block copy still active: domain has active 
block job

Of course we tried to start/stop the virtual machines, rebooted the 
whole host multiple times etc., but the problem comes back every night.

The machine is a Debian Wheezy machine with current updates. We are 
using the qemu-kvm package from wheezy-backports to enable blockcopy 
support.

Best regards
Markus