doc: no info on how to log in into a qemu instance

Following the doc and the command under the `Run an emulated ARM64 VM on x86` section, I am presented with a login prompt and user:ubuntu pass:ubuntu is not accepted.

Doc should mention how to log in into that instance.

```
[ 744.226683] cloud-init[1248]: Cloud-init v. 24.3.1-0ubuntu0~22.04.1 finished at Tue, 26 Nov 2024 13:16:12 +0000. Datasource DataSourceNone. Up 744.15 seconds
[ 744.234976] cloud-init[1248]: 2024-11-26 13:16:12,170 - cc_final_message.py[WARNING]: Used fallback datasource
[ OK ] Finished Cloud-init: Final Stage.
[ OK ] Reached target Cloud-init target.

ubuntu login: ubuntu
Password:

Login incorrect
```

---
Document: [how-to/virtualisation/arm64-vms-on-qemu.md](https://documentation.ubuntu.com/server/how-to/virtualisation/arm64-vms-on-qemu/#run-an-emulated-arm64-vm-on-x86)

Start with how-to/virtualisation/arm64-vms-on-qemu.md and read the “Run an emulated ARM64 VM on x86” section and its command. Reproduce the login prompt if possible, then update that section with the correct login instructions. Done means a reader can determine how to log in to the instance without guessing credentials.

Written by the indexing model from the issue text.