console: add graphic_hw_update_done()

console: add graphic_hw_update_done()

Add a function to be called when a graphic update is done.

Declare the QXL renderer as async: render_update_cookie_num counts the
number of outstanding updates, and graphic_hw_update_done() is called
when it reaches none.

(note: this is preliminary work for asynchronous screendump support)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>