Testing and documentation updates: - bump avocado to 101.0 - use snapshots for tuxrun baseline tests - add sbda-ref test to avocado - avoid spurious re-configure in gitlab - better description of blockdev options - drop FreeBSD 12 from Cirrus CI - fix up the ast2[56]00 tests to be more stable - improve coverage of ppc64 tests in tuxrun baselines - limit plugin tests to just the generic multiarch binaries