[XPU] use `ZE_AFFINITY_MASK` for device select on xpu (#21815)

Signed-off-by: Kunshang Ji <kunshang.ji@intel.com>

vllm/platforms/xpu.py

@@ -30,7 +30,7 @@ class XPUPlatform(Platform):
     # see https://github.com/ray-project/ray/blob/6a5eb5865eeb9ccf058a79b44f107e327e360673/python/ray/_private/accelerators/intel_gpu.py#L20 # noqa: E501
     ray_device_key: str = "GPU"
     dist_backend: str = "ccl"  # ccl | xccl
-    device_control_env_var: str = "ONEAPI_DEVICE_SELECTOR"
+    device_control_env_var: str = "ZE_AFFINITY_MASK"
 
     @classmethod
     def get_attn_backend_cls(cls, selected_backend: _Backend, head_size: int,