feat(lxd): add s390x virtio-ports detection for LXD

[blackboxsw]

• [x] I have signed the CLA: https://ubuntu.com/legal/contributors
• [x] I have included a comprehensive commit message using the guide below
• [x] I have added unit tests to cover the new behavior under tests/unittests/
  • Test files should map to source files i.e. a source file cloudinit/example.py should be tested by tests/unittests/test_example.py
  • Run unit tests with tox -e py3
• [x] I have kept the change small, avoiding unnecessary whitespace or non-functional changes.
• [x] I have added a reference to issues that this PR relates to in the PR message (Refs GH-1234, Fixes GH-1234)
• [x] I have updated the documentation with the changed behavior.
  • If the change doesn't change the user interface and is trivial, this step may be skipped.
  • Cloud-config documentation is generated from the jsonschema.
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Proposed Commit Message

feat(lxd): add s390x virtio-ports detection for LXD

Support LXD detection on IBM's s390x architecture using virtio-ports
serial device detection. Add a new detection method which checks
LXD serial devices in /sys/class/virtio-ports/ for both current
(com.canonical.lxd) and legacy (org.linuxcontainers.lxd) names.
Update public LXD datasource documentation for this detection method.

Drop the now unnecessary DMI board name check from ds-identify and
DataSourceLXD because virtio-ports is sufficient for both KVM/QEMU
and s390x.

The following mechinisms for LXD detection are retained:
1. /dev/lxd/sock socket file (primary method)
2. virtio-ports serial device presence if socket is not yet active

Implements: PL073

Additional Context

Test Steps

git checkout upstream/ubuntu/questing -B ubuntu/questing
new_upstream_snapshot -c 5ff9034ffdac4a59a871384c1bd082c2589a1c4e
build-package
sbuild ....
CLOUD_INIT_OS_IMAGE=questing CLOUD_INIT_CLOUD_INIT_SOURCE=cloud-init_25.4~2gdef5afa3-0ubuntu1_all.deb CLOUD_INIT_PLATFORM=lxd_vm .tox/integration-tests/bin/pytest tests/integration_tests/datasources/test_lxd_discovery.py::test_lxd_kvm_datasource_discovery_without_lxd_socket

Merge type

• [x] Squash merge using "Proposed Commit Message"
• [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[blackboxsw]

Additionally, we should add resolute support to pycloudlib per https://github.com/canonical/pycloudlib/pull/494