Bug#989462: Info received (About bumping Vagrant box disk image to 1TB)

[Hans-Christoph Steiner <hans@eds.org>]

> - you add to your pull request a change of the virtualized disk
> controller from virtio-blk to virtio-scsi and to the default libvirt
> vagrantfile the "unmap" option so that deletion of blocks in the guest
> are propagated om host storage

I looked into this a bit more.  These feature weren't added to vagrant-libvirt 
until 0.4.0, so no stable Debian package of vagrant-libvirt could support this 
yet.  Debian/testing has 0.7.0, and a backport to stable would be possible.  I 
guess a lot of people just use `vagrant plugin install ...`. It might be 
possible to make it work with older vagrant-libvirt versions by using a hack 
like this:

https://github.com/vagrant-libvirt/vagrant-libvirt/pull/692#issuecomment-922329049


There is a little more info here:
https://github.com/vagrant-libvirt/vagrant-libvirt/issues/999#issuecomment-487728207


```
ENV['VAGRANT_EXPERIMENTAL'] = 'typed_triggers'

require 'open3'

Vagrant.configure('2') do |config|
  ...
  config.vm.provider :libvirt do |lv, config|
    lv.storage :file, :size => '3G', :device => 'sda', :bus => 'scsi', :discard 
=> 'unmap', :cache => 'unsafe'
    config.trigger.before 
:'VagrantPlugins::ProviderLibvirt::Action::StartDomain', type: :action do |trigger|
      trigger.ruby do |env, machine|
        stdout, stderr, status = Open3.capture3(
          'virt-xml', machine.id,
          '--edit', 'type=scsi',
          '--controller', 'model=virtio-scsi')
        if status.exitstatus != 0
          raise "failed to run virt-xml to modify the controller model. 
status=#{status.exitstatus} stdout=#{stdout} stderr=#{stderr}"
        end
      end
    end
    ...
  end
end
```