hw/nvme: skip queue processing if notifier is cleared

While it is safe to process the queues when they are empty, skip it if
the event notifier callback was invoked spuriously.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Jinhao Fan <fanjinhao21s@ict.ac.cn>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>