UPSTREAM PR #19204: ggml-virtgpu: make the code thread safe

[loci-dev]

Note

Source pull request: ggml-org/llama.cpp#19204

This PR improves the code of the ggml-virtgpu backend to make it thread safe, by using mutex for accessing the host<>guest shared memory buffers, and by pre-caching, during the initialization, the constant values queried from the backend.

The unused buffer_type_is_host method is also deprecated.

[loci-review]

No meaningful performance changes were detected across 115126 analyzed functions in the following binaries: build.bin.llama-tts, build.bin.llama-cvector-generator, build.bin.libllama.so, build.bin.libmtmd.so, build.bin.llama-tokenize, build.bin.llama-quantize, build.bin.llama-qwen2vl-cli, build.bin.libggml-base.so, build.bin.libggml-cpu.so, build.bin.libggml.so, build.bin.llama-bench, build.bin.llama-gemma3-cli, build.bin.llama-gguf-split, build.bin.llama-llava-cli, build.bin.llama-minicpmv-cli.

🔎 Full breakdown: Loci Inspector.
💬 Questions? Tag @loci-dev.

[loci-review]

No meaningful performance changes were detected across 115126 analyzed functions in the following binaries: build.bin.llama-tts, build.bin.llama-cvector-generator, build.bin.libllama.so, build.bin.libmtmd.so, build.bin.llama-gguf-split, build.bin.llama-llava-cli, build.bin.llama-minicpmv-cli, build.bin.llama-quantize, build.bin.llama-gemma3-cli, build.bin.llama-tokenize, build.bin.llama-qwen2vl-cli, build.bin.llama-bench, build.bin.libggml-base.so, build.bin.libggml-cpu.so, build.bin.libggml.so.

🔎 Full breakdown: Loci Inspector.
💬 Questions? Tag @loci-dev.