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
7 jobs for fstrim in 60 minutes and 29 seconds (queued for 2 seconds)
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