[XPU][NIXL] Support GPUDirect RDMA for KV cache transfers

[Copilot]

Enables GPUDirect RDMA for Intel XPU devices in the NIXL KV transfer connector, allowing direct device-to-device KV cache transfers without staging through host memory.

Changes

• nixl_connector.py: Add "xpu" to supported KV buffer devices so XPU device memory is used directly for NIXL transfers instead of CPU buffers.
• vllm/platforms/xpu.py: Set UCX_MEMTYPE_CACHE=n when kv_transfer_config is enabled to prevent UCX from misdetecting XPU device memory as host memory. Scoped to KV transfer path only — no effect on non-KV-transfer XPU workloads.
• tools/install_nixl_from_source_ubuntu.py: Pin UCX to commit e5d9887 and enable --with-ze at configure time. This is the first UCX revision with Intel Level Zero GPU memory registration support required for XPU GDR. Documented with guidance for future hash updates.

Performance

Llama3.3-70B int4, fp8 KV cache, 8×B60, ISL=1500, OSL=150 — under SLO (TTFT<5s, ITL<100ms):

Config	Requests/sec
Non-PD (tp=2 × 3 round-robin)	0.64
2P1D (NIXL XPU GDR)	1.06 — 1.65× improvement

Prerequisite

Requires UCX built from commit e5d9887 or later with --with-ze. The install script handles this automatically.

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