virgl: Introduce and use virgl_staging_mgr
Add a manager for the staging buffer used in virgl. The staging manager is heavily inspired by u_upload_mgr, but is simpler and is a better fit for virgl's purposes. In particular, the staging manager:
- Allows concurrent staging allocations.
- Calls virgl directly to create map resources, avoiding unnecessarily going through gallium resources and transfers.
Signed-off-by: Alexandros Frantzis email@example.com