hw/nvme: do not enable ioeventfd by default

Do not enable ioeventfd by default. Let the feature mature a bit before
we consider enabling it by default.

Fixes: 2e53b0b4 ("hw/nvme: Use ioeventfd to handle doorbell updates")
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Jinhao Fan <fanjinhao21s@ict.ac.cn>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>