blockdev: Fix wrong usage of QDECREF causing snapshoted quorum to crash on close.

As bdrv_open() documentation states:
"The reference to the QDict belongs to the block layer
 * after the call (even on failure), so if the caller intends to reuse the
 * dictionary, it needs to use QINCREF() before calling bdrv_open."

the optional options dict will not be reused after bdrv_open() and should
belong to the block layer so remove the extra QDECREF(options).

Signed-off-by: Benoit Canet <benoit@irqsave.net>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

blockdev.c

@@ -1329,8 +1329,6 @@ static void external_snapshot_prepare(BlkTransactionState *common,
     if (ret != 0) {
         error_propagate(errp, local_err);
     }
-
-    QDECREF(options);
 }
 
 static void external_snapshot_commit(BlkTransactionState *common)