Split virtualization hardware requirement into per-sub-requirement rows in the support table

[thrix]

Problem

The hardware requirement support table shows virtualization as a single row with footnotes explaining partial support. This is confusing because it implies full support when only is-virtualized and hypervisor are implemented by beaker and artemis. The is-supported and confidential sub-requirements have no provisioner support at all.

Raised during review of #4638 by @LecrisUT: #4638 (comment)

Proposed solution

Split spec/hardware/virtualization.fmf into a directory with separate child nodes, each with their own implemented-by links:

spec/hardware/virtualization/
├── main.fmf              # parent node with shared summary
├── hypervisor.fmf         # implemented-by: artemis, mrack
├── is-virtualized.fmf     # implemented-by: artemis, mrack
├── is-supported.fmf       # no implemented-by
└── confidential.fmf       # no implemented-by

The table generator (docs/ext/generate_hardware_matrix.py) already creates one row per /spec/hardware/* story, so the split would automatically produce separate rows showing exactly which provisioner supports which sub-requirement - no footnotes needed.

Notes

• Other specs with sub-properties (cpu, disk) have the same pattern and could benefit from similar splitting, but that's a larger effort.
• The virtualization case is the most confusing because some sub-requirements have zero provisioner support.