Fix/kvm disk image corruption

Tasks:

• merge disk-image-corruption present in instance-kvm.cfg.jinja2 and instance-kvm-import.cfg.jinja2.in
• make it report information on failure
• assure it's being run in short time, by executing it with cron and sharing the status
  • it is short time, see:

du -sh srv/virtual.qcow2 ; time ./software_release/parts/qemu/bin/qemu-img check -U srv/virtual.qcow2
292G    srv/virtual.qcow2
No errors were found on the image.
4782342/4915200 = 97.30% allocated, 6.82% fragmented, 0.00% compressed clusters
Image end offset: 313466159104

real    0m0.801s
user    0m0.659s
sys     0m0.164s

• consider speed improvements
  • thus none needed