ds-identify: detect LXD for VMs launched from host with > 5.10 kernel

[blackboxsw]

Proposed Commit Message

Launching KVM instances from a host system with > 5.10 kernel results
in LXC passing `hv_passthrough` to the kvm instance being launched.
Systemd < 251 will incorrectly detect the CPU in this case as
"qemu" instead of "kvm".

ds-identify needs to properly interpret systems with
sytemd-detect-virt="qemu"  and /sys/class/dmi/id/board_name="LXD"

This functionality can be dropped once systemd 251 support is
available on all supported distributions.

LP: #1968085

Additional Context

systemd/systemd#22709

Test Steps

From a jammy host:

$ lxc profile show jammy-vm
config: {}
description: Default LXD profile for jammy VMs
devices:
  eth0:
    name: eth0
    network: lxdbr0
    type: nic
  root:
    path: /
    pool: default
    type: disk
name: jammy-vm

$ lxc launch ubuntu-daily:jammy --vm --profile jammy-vm test-jammy
$ lxc exec test-jammy -- systemd-detect-virt
qemu
$ lxc stop test-jammy
$ lxc config set test-jammy security.devlxd=false
$ lxc start test-jammy
$ lxc exec test-jammy -- DEBUG_LEVEL=2 DI_LOG=stderr /usr/lib/cloud-init/ds-identify --force
# Ensure "no datasource detected"
$ lxc exec test-jammy -- add-apt-repository ppa:cloud-init-dev/daily
$ lxc exec test-jammy -- apt install cloud-init
$ lxc exec test-jammy -- DEBUG_LEVEL=2 DI_LOG=stderr /usr/lib/cloud-init/ds-identify --force
"Found single datasource: LXD"

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[holmanb]

This functionality can be dropped once systemd 251 support is
available.

I don't think this is true, is it? There may be distros/releases out there that might be stuck on a kernel version > 5.10 with a systemd version < 250 for quite some time. Without auditing all supported distros I don't think we can drop this check as soon as 251 is available.

Also I see a format issue that will likely fail on CI shortly (line length).

[blackboxsw]

This functionality can be dropped once systemd 251 support is
available.

I don't think this is true, is it? There may be distros/releases out there that might be stuck on a kernel version > 5.10 with a systemd version < 250 for quite some time. Without auditing all supported distros I don't think we can drop this check as soon as 251 is available.

Also I see a format issue that will likely fail on CI shortly (line length).

Fixed and updated both PR "Proposed Commit Message" in this description and the test_ds_identify docstr awaiting CI pass before we can merge.