The new _ASYNC io's in qxl_dev listed at the end get six new api
functions, and an additional callback function "async_complete". When
the async version of a specific io is used, completion is notified by
calling async_complete, and no READY message is written or expected by
the dispatcher.
update_area has been changed to push QXLRects to the worker thread, where
the conversion to SpiceRect takes place.
A cookie has been added to each async call to QXLWorker, and is passed back via
async_complete.
Added api:
QXLWorker:
update_area_async
add_memslot_async
destroy_surfaces_async
destroy_primary_surface_async
create_primary_surface_async
destroy_surface_wait_async
QXLInterface:
async_complete