Chnage vm-target renderer to software renderer pixman

[juliuskoskela]

Description of changes

The virtual machine target packages.x86_64-linux.vm-debug wouldn't boot after we have integrated hardware rendering. Changed this setting to software renderer "pixman" for now so that we get a clean boot from running this target.

Checklist for things done

• [X ] Summary of the proposed changes in the PR description
• [ X] More detailed description in the commit message(s)
• [X ] Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• [X ] Contribution guidelines followed
• [ X] Test procedure described (or includes tests). Select one or more:
  • [X ] Tested on Lenovo X1 x86_64

Instructions for Testing

Run Ghaf in vm according to instructions at https://tiiuae.github.io/ghaf/ref_impl/build_and_run.html#running-ghaf-image-for-x86-vm-ghaf-host