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>