virtio_blk: fix race at module removal

If a device appears while module is being removed,
driver will get a callback after we've given up
on the major number.

In theory this means this major number can get reused
by something else, resulting in a conflict.

To fix, cleanup in reverse order of initialization.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>