hw/nvme: Add trace events for shadow doorbell buffer

hw/nvme: Add trace events for shadow doorbell buffer

When shadow doorbell buffer is enabled, doorbell registers are lazily
updated. The actual queue head and tail pointers are stored in Shadow
Doorbell buffers.

Add trace events for updates on the Shadow Doorbell buffers and EventIdx
buffers. Also add trace event for the Doorbell Buffer Config command.

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