blockdev: check for BLOCK_OP_TYPE_INTERNAL_SNAPSHOT

The BLOCK_OP_TYPE_INTERNAL_SNAPSHOT op blocker exists but was never
used!  Let's fix that so internal snapshots can be blocked.

[Fixed s/external/internal/ typo as pointed out by Paolo Bonzini and Max
Reitz.
--Stefan]
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 1416566940-4430-5-git-send-email-stefanha@redhat.com
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1250,6 +1250,10 @@ static void internal_snapshot_prepare(BlkTransactionState *common,
         return;
     }
 
+    if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT, errp)) {
+        return;
+    }
+
     if (bdrv_is_read_only(bs)) {
         error_set(errp, QERR_DEVICE_IS_READ_ONLY, device);
         return;