DataSourceEc2: all-zero /sys/hypervisor/uuid on Xen dom0 breaks AWS detection and IMDSv2

# Bug report
On a Xen dom0 host, `/sys/hypervisor/uuid` reads all zeros.
`_collect_platform_data()` prefers that file over DMI, all zeros is truthy,
so the DMI fallback never runs and AWS is not identified
([DataSourceEc2.py lines 878-882 at aad14e5d](https://github.com/canonical/cloud-init/blob/aad14e5d8f0343dd39e434715692f2e2a6f4cf9f/cloudinit/sources/DataSourceEc2.py#L878-L882)):

```python
uuid = None
with suppress(OSError, UnicodeDecodeError):
uuid = util.load_text_file("/sys/hypervisor/uuid").strip()

uuid = uuid or dmi.read_dmi_data("system-uuid") or ""
```

```
$ cat /sys/hypervisor/uuid
00000000-0000-0000-0000-000000000000
$ cat /sys/class/dmi/id/product_uuid
ec2... (the DMI fallback would have identified AWS)
```

`_maybe_fetch_api_token()` then returns early
([lines 301-310](https://github.com/canonical/cloud-init/blob/aad14e5d8f0343dd39e434715692f2e2a6f4cf9f/cloudinit/sources/DataSourceEc2.py#L301-L310)),
so no IMDSv2 token is requested; with `HttpTokens=required` the tokenless
IMDSv1 requests fail for the full 240 s wait and the datasource gives up.

## Steps to reproduce the problem
1. Launch a stock Ubuntu 24.04 EC2 instance with
`--metadata-options HttpTokens=required`.
2. Install Xen (dom0 is PV, so no nested virtualization is needed), make it
the grub default, reboot:

```
sudo apt-get update && sudo apt-get install -y xen-system-amd64
echo 'GRUB_DEFAULT="Ubuntu GNU/Linux, with Xen hypervisor"' | sudo tee /etc/default/grub.d/xen.cfg
sudo update-grub && sudo reboot
```

3. On the Xen boot, cloud-init retries the tokenless IMDSv1 path for 240 s,
then falls back to DataSourceNone, regenerating the hostname and SSH
host keys.

## Environment details
- Cloud-init version: 26.1-0ubuntu1~24.04.1. Code unchanged on main at
aad14e5d.
- Operating System Distribution: Ubuntu 24.04 booted as Xen dom0
(xen-system-amd64, m5.large)
- Cloud provider, platform or installer type: AWS EC2 with
`HttpTokens=required`

## cloud-init logs
Serial console from the repro (note the tokenless legacy path):

```
[ 8.5] url_helper.py[WARNING]: Calling 'http://169.254.169.254/2009-04-04/meta-data/instance-id' failed [0/240s]: ...
...
[ 244.1] url_helper.py[WARNING]: Calling 'http://169.254.169.254/2009-04-04/meta-data/instance-id' failed [235/240s]: ...
[ 244.1] DataSourceEc2.py[CRITICAL]: Giving up on md from ['http://169.254.169.254/2009-04-04/meta-data/instance-id', ...] after 235 seconds
[ 244.1] DataSourceEc2.py[ERROR]: Unable to get metadata
[ 257.6] Cloud-init v. 26.1-0ubuntu1~24.04.1 finished ... Datasource DataSourceNone. Up 257.66 seconds
[ 257.6] cc_final_message.py[WARNING]: Used fallback datasource
```

Start in cloudinit/sources/DataSourceEc2.py at _collect_platform_data() lines 878-882, then read _maybe_fetch_api_token() around lines 301-310. Trace how an all-zero Xen UUID prevents the DMI fallback and stops token retrieval; done means AWS is detected and IMDSv2 token requests proceed on the reported Xen dom0 setup.

Written by the indexing model from the issue text.