Add support for Juniper vRR 20.4R3.8-KVM

Before submitting a pull request, please check the following.

When updating an existing appliance:

[ ] The new version is on top.
[ ] The filenames in the "images" section are unique, to avoid appliances / version overwriting each other.
[ ] If you forked the repo, running check.py doesn't drop any errors for the updated file.

When creating a new appliance:
It's tested locally, i.e.
- [ ] You dragged an instance into a project on your box, got it installed (if necessary), and did some basic network checks (ping, UI reachable, etc.).
- [ ] GNS3 VM can run it without any tweaks.
- [ ] The device is in the right category: router, switch, guest (hosts), firewall
- [ ] You filled in as much info as possible (checks the schemas and other appliance files for some guidance).
[ ] When adding a container: it builds on Docker Hub and can be pulled.
[ ] The filenames in the "images" section are unique (to avoid appliances and/or versions overwriting each other).
[ ] If you forked the repo, running check.py doesn't drop any errors for the new file.
[ ] Optional: a symbol has been created for the new appliance.

GNS3 / gns3-registry