virgl: Introduce virgl_staging_mgr

virgl: Introduce 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 the virgl winsys directly to create and map resources, avoiding
  unnecessarily going through gallium resources and transfers.

olv: make virgl_staging_alloc_buffer return a bool

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>