qemu-io: Use bdrv_drain_all instead of qemu_aio_flush

This is harmless as of today because I/O throttling is not used in
qemu-io, however as soon as .bdrv_drain handlers will be introduced,
qemu-io must be sure to call bdrv_drain_all().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-io.c

@@ -1362,7 +1362,7 @@ static int aio_write_f(int argc, char **argv)
 
 static int aio_flush_f(int argc, char **argv)
 {
-    qemu_aio_flush();
+    bdrv_drain_all();
     return 0;
 }