The manifest uses hyphenated keys (guest-agents, hardware-support) but the filename
generation was looking for underscored keys, causing 16 variants to overwrite 4 files.
Added _get_modifier() helper to normalize both key formats and added _bare-metal suffix
for False values of guest_agents, ensuring unique filenames for all variant combinations.
This restores the intended 24 distinct variants from the manifest and adds support for
all modifiers (bootloader, initial-setup, desktop, security, storage, guest_agents, hardware_support).
