vdpa: Add documentation for vdpa_alloc_device() macro

The return value of vdpa_alloc_device() macro is not very
clear, so that most of callers did the wrong check. Let's
add some comments to better document it.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210715080026.242-4-xieyongji@bytedance.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>