throttle: add missing coroutine_fn annotations

Callers of coroutine_fn must be coroutine_fn themselves, or the call
must be within "if (qemu_in_coroutine())".  Apply coroutine_fn to
functions where this holds.

Reviewed-by: Alberto Faria <afaria@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220922084924.201610-20-pbonzini@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/throttle.c

@@ -162,7 +162,7 @@ static int coroutine_fn throttle_co_pwritev_compressed(BlockDriverState *bs,
                                BDRV_REQ_WRITE_COMPRESSED);
 }
 
-static int throttle_co_flush(BlockDriverState *bs)
+static int coroutine_fn throttle_co_flush(BlockDriverState *bs)
 {
     return bdrv_co_flush(bs->file->bs);
 }