venus: prefer VIRTGPU_BLOB_MEM_HOST3D for shmems
Prefer HOST3D over GUEST for shmems.
This depends on virgl/virglrenderer!649 (merged). It makes shmems memfd-based rather than iov-based, which allows them to be shared in case of process isolation.
This can also be desirable without process isolation, because it makes shmems logically contiguous in the host.
Edited by Chia-I Wu