Improve disk compaction by trimming the disk

Merged Kristian Klausen requested to merge klausenbusk/arch-boxes:fstrim into try-software-mode
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 <- before
425M <- after

Fix #106 

I haven't been able to test this with VirtualBox as it crash (guru meditation).

I did consider removing the write_zeroes logic, but it could still be useful for VMware users. Should we remove it?

Edited by Kristian Klausen

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