bug & fix: ModelVBAR crashes on NVIDIA GRID / vGPU instead of falling back

Hey,
i ran into a MemoryError on my setup even though I have enough VRAM.
Took a while to figure out but the issue is that VBAR allocation just fails silently on virtualized GPUs (vGPU environments) and then throws a hard exception instead of falling back gracefully.

`MemoryError: VBAR allocation failed
File "execution.py", line 525, in execute
File "execution.py", line 334, in get_output_data
File "custom_nodes/rgthree-comfy/py/power_prompt_simple.py", line 39, in main
conditioning = CLIPTextEncode().encode(opt_clip, prompt)[0]
File "nodes.py", line 80, in encode
return (clip.encode_from_tokens_scheduled(tokens), )
File "comfy/sd.py", line 375, in encode_from_tokens
self.load_model(tokens)
File "comfy/model_management.py", line 819, in load_models_gpu
loaded_model.model_load(lowvram_model_memory, ...)
File "comfy/model_patcher.py", line 1495, in _vbar_get
vbar = comfy_aimdo.model_vbar.ModelVBAR(self.model_size() * 10, self.load_device.index)
File "comfy_aimdo/model_vbar.py", line 51, in __init__
raise MemoryError("VBAR allocation failed")`

My setup: GRID RTX6000-24Q (24GB vGPU), Ubuntu, CUDA 12.8
VBAR cannot allocate because the underlying low-level CUDA memory ops aren't fully supported in virtualized environments.

### What I changed (i got help from AI to find a working fix)

- `ModelVBAR.__init__` no longer raises `MemoryError` when allocation fails, instead sets `_disabled = True` and emits a `RuntimeWarning`
- All methods (`alloc`, `fault`, `prioritize`, `deprioritize`, `loaded_size`, etc.) check `_disabled` and return safe dummy values instead of crashing
- `vbar_fault()` and `vbar_unpin()` module-level functions also handle `None` alloc gracefully now

ComfyUI runs fine without VBAR on these setups anyway since the optimization doesn't really work in a virtualized context, so this just makes it degrade nicely instead of hard crashing.

Or do i miss something else?

Start with comfy_aimdo/model_vbar.py and the ModelVBAR call in comfy/model_patcher.py shown in the traceback. Trace the allocation failure through ModelVBAR methods and the vbar_fault() and vbar_unpin() helpers. Done means the NVIDIA vGPU path no longer raises MemoryError and ComfyUI continues without VBAR, while normal allocation behavior remains intact.

Written by the indexing model from the issue text.