Add UEFI variable append tests

[dinhngtu]

Fallout from the varstored update.

At the moment, test the following scenarios:

• From the varstored defaults, append MS dbx and verify VM boots
• From the varstored 1.2.0-3.1 defaults, append MS dbx and verify VM boots (which implies not having the oversized variable append bug)

[guzu]

I don't understand all of that, but just wonder if part of the message in commit log could make sense as a comment in test ?
Mainly "which implies not having the oversized variable append bug", maybe that's what test_append_with_poison() is just about.

[dinhngtu]

The context is here: https://xcp-ng.org/blog/2025/10/30/xcp-ng-8-3-varstored-update-unbootable-vm-risk-and-remediation/

In short, the dbx variable previously used in the bad update did not use the Microsoft owner GUID, preventing deduplication of EFI signature data entries during an append call. Normally, this would not cause the VM to crash; except varstored does not check for the variable data length on append (https://redirect.github.com/xapi-project/varstored/commit/46edc9d071bdee42a80b312540c0fc076c227db0, backported but still not released), triggering the issue.