jenkins-setup-build-environment: delete libvirt images before test run
jenkins-setup-build-environment: delete libvirt images before test run

profitbricks-build7-amd64 was running out of disk space when running this
job...

f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver