virtio-blk: support per-device queue depth

module parameter 'virtblk_queue_depth' was firstly introduced for
testing/benchmarking purposes described in commit fc4324b4
("virtio-blk: base queue-depth on virtqueue ringsize or module param").
And currently 'virtblk_queue_depth' is used as a saved value for the
first probed device.
Since we have different virtio-blk devices which have different
capabilities, it requires that we support per-device queue depth instead
of per-module. So defaultly use vq free elements if module parameter
'virtblk_queue_depth' is not set.

Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/1611307306-71067-1-git-send-email-joseph.qi@linux.alibaba.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>