isobuilders can't start Vagrant VMs => all ISO builds fail

Originally created by @intrigeri on #16370 (Redmine)

See e.g. /var/log/libvirt/qemu/tails-builder-amd64-stretch-20190114-e426707444_default.log on isobuilder1.lizard.

I’ve verified that /sys/module/kvm_intel/parameters/nested says Y on lizard.