/var/lib/libvirt/images sometimes gets filled on isobuilders, take 2

Originally created by @intrigeri on #13302 (Redmine)

icinga2 just told me that this problem is not 100% solved, and just happened again on isobuilder1 during https://jenkins.tails.boum.org/job/reproducibly_build_Tails_ISO_devel/27/:

$ ls -l /var/lib/libvirt/images
total 2.8G
-rw------- 1 libvirt-qemu libvirt-qemu 1.5G Jun 30 16:46 tails-builder-amd64-jessie-20170609-daf786c121_default.img
-rwxr--r-- 1 libvirt-qemu libvirt-qemu 1.3G Jun 30 15:02 tails-builder-amd64-jessie-20170609-daf786c121_vagrant_box_image_0.img*
$ df /var/lib/libvirt/images
/dev/vdc    ext4      2.9G  2.8G     0 100% /var/lib/libvirt/images

Parent Task: tails/sysadmin#12531

Related issues

• Related to tails/sysadmin#12599
• Blocks tails/sysadmin#12618