Misc cleanups
Hi,
v2:
- drop "compiler.h: add QEMU_{BEGIN,END}_IGNORE_INITIALIZER_OVERRIDES",
"qobject/json-lexer: disable -Winitializer-overrides warnings" &
"qapi/error: add g_autoptr(Error) support" and adjust related code.
- add "test/qga: use g_auto wherever sensible"
- add r-b tags