docs: Partial toolstack library API/ABI stabilisation

docs: Partial toolstack library API/ABI stabilisation

---
v10: Resync updates made throughout v9 of the series into a new
baseline.

DraftB:
  - Feedback from Jan and Andy
  - QEMU usage by functional area
  - Annotated with what things are and users
  - Renamed to docs/misc/toolstack-library-abis.pandoc
  - Change method for gathering symbols to omit static/non-exported
    stuff.
  - Some additional functional classification, and start to annotate
    the underlying interfaces plus known users.