Move qapi-schema.json to qapi/, so it's next to its modules, and all
files get generated to qapi/, not just the ones generated for modules.
Consistently name the generated files qapi-MODULE.EXT:
qmp-commands.[ch] become qapi-commands.[ch], qapi-event.[ch] become
qapi-events.[ch], and qmp-introspect.[ch] become qapi-introspect.[ch].
This gets rid of the temporary hacks in scripts/qapi/commands.py,
scripts/qapi/events.py, and scripts/qapi/common.py.
Backports commit eb815e248f50cde9ab86eddd57eca5019b71ca78 from qemu
