Display management in gui-vm

[gngram]

#Display management in gui-vm

Description

Start display-suspend.service on gui-vm when there suspend.target is triggered in host.
Start display-resume.service on gui-vm when there sleep.target is triggered in host.
Updated admin test.

Checklist

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Test procedure added to nixos/tests
• Author has run nix flake check --accept-flake-config and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing

Successfully executed admin vm tests.

[avnik]

Basically looks good, I added few minors comment