Improve disk compaction by trimming the disk
This only affect the qemu and virtualbox image. I'm not sure how you enable trim support in VMware and the documentation is spare. Tested with: packer build -only=qemu local.json 698M Arch-Linux-x86_64-libvirt-2020-08-01.box <- before 425M Arch-Linux-x86_64-libvirt-2020-08-01.box <- after Fix #106