Add G_GNUC_UNUSED annotations to async_read_handler args
Add G_GNUC_UNUSED annotations to async_read_handler args

2 of the arguments are not used, the G_GNUC_UNUSED annotation will make
this explicit.

elmarco/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,fgouget/spice,ya-mouse/spice,elmarco/spice,SPICE/spice,Fantu/spice,Fantu/spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,Fantu/spice,fgouget/spice,SPICE/spice,elmarco/spice,ya-mouse/spice,Fantu/spice,elmarco/spice