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)
latest
Status Name Job ID Coverage
  Lint
passed flake8 #2467

00:00:12

passed shellcheck #2464

00:00:25

passed shfmt #2465

00:00:23

passed validate-packer #2468

00:00:26

passed yapf #2466

00:00:22

 
  Build
failed build:cloud-qemu #2469

01:00:00

failed build:vagrant-qemu #2470

01:00:00

 
Name Stage Failure
failed
build:vagrant-qemu Build The script exceeded the maximum execution time set for the job
    qemu: view the screen of the VM, connect via VNC without a password to
qemu: vnc://127.0.0.1:5909

==> qemu: Overriding defaults Qemu arguments with QemuArgs...
==> qemu: Waiting 1m0s for boot...
==> qemu: Connecting to VM via VNC (127.0.0.1:5909)
==> qemu: Typing the boot command over VNC...
==> qemu: Using ssh communicator to connect: 127.0.0.1
==> qemu: Waiting for SSH to become available...
ERROR: Job failed: execution took longer than 1h0m0s seconds
failed
build:cloud-qemu Build The script exceeded the maximum execution time set for the job
    Arch-Linux-x86_64-cloudimg-2020-08-13.img: view the screen of the VM, connect via VNC without a password to
Arch-Linux-x86_64-cloudimg-2020-08-13.img: vnc://127.0.0.1:5954

==> Arch-Linux-x86_64-cloudimg-2020-08-13.img: Overriding defaults Qemu arguments with QemuArgs...
==> Arch-Linux-x86_64-cloudimg-2020-08-13.img: Waiting 1m0s for boot...
==> Arch-Linux-x86_64-cloudimg-2020-08-13.img: Connecting to VM via VNC (127.0.0.1:5954)
==> Arch-Linux-x86_64-cloudimg-2020-08-13.img: Typing the boot command over VNC...
==> Arch-Linux-x86_64-cloudimg-2020-08-13.img: Using ssh communicator to connect: 127.0.0.1
==> Arch-Linux-x86_64-cloudimg-2020-08-13.img: Waiting for SSH to become available...
ERROR: Job failed: execution took longer than 1h0m0s seconds