tweaks and fixes for 8.0-rc1 (tests, plugins, docs) As usual for softfreeze I switch from my usual maintainer trees to collecting up miscellaneous fixes and tweaks as we stabilise the build. Currently these are still mostly testing focused although there is a plugins fix I forgot to send a PR for earlier. The following need review: - tests/avocado: don't use tags to define drive - include/exec: fix kerneldoc definition - tests/tcg: disable pauth for aarch64 gdb tests - tests/tcg: add some help output for running individual tests - include/qemu: add documentation for memory callbacks - gitlab: update centos-8-stream job - scripts/ci: update gitlab-runner playbook to handle CentOS - tests/docker: all add DOCKER_BUILDKIT to RUNC environment