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