Add private data field to struct virgl_renderer_resource_create_args

Please add a field to strucr virgl_renderer_resource_create_args to store some user data (e.g. void *priv). The value of the filed would be later returned within the struct virgl_renderer_resource_info upon a call to virgl_renderer_resource_get_info().

This will simplify usage of the library and avoid extra complication in various use cases. For instance, I have to maintain a separate resource tracker and store all the virgl resource info along with my own data because I cannot really make use of the virglrenderer's built in one.