stubdom: fix read-only disks access

There is no need for a flush on read-only disks.  It would actually
even error out and disturb the guest.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>

tools/ioemu/block-vbd.c

@@ -273,6 +273,10 @@ static BlockDriverAIOCB *vbd_aio_flush(BlockDriverState *bs,
     BDRVVbdState *s = bs->opaque;
     VbdAIOCB *acb = NULL;
 
+    if (s->info.mode == O_RDONLY) {
+        cb(opaque, 0);
+        return NULL;
+    }
     if (s->info.barrier == 1) {
         acb = vbd_aio_setup(bs, 0, NULL, 0,
                 s->info.flush == 1 ? vbd_nop_cb : cb, opaque);