fix(vmm): restrict listed GPUs to the node's offered devices

[kvinwang]

Problem

With attach_mode = "listed", resolve_gpus copies each requested slot straight into -device vfio-pci,host={slot} without checking cvm.gpu.listing / include / exclude. Any host PCI device is attachable, including one the operator excluded or a non-GPU, and the slot string is never validated (QEMU treats , and = as option separators).

Fix

resolve_gpus_with_config rejects a listed slot that is not in GpuConfig::list_devices() — the same set ListGpus returns to the web UI picker. Applies to both CreateVm and UpdateVm.

Compat: the web UI can only pick offered slots, so it is unaffected; vmm-cli --gpu requests naming an unoffered slot are now rejected. Costs one lspci per listed-GPU request.

Split out of #1286.

Verification

cargo test -p dstack-vmm (210 passed), cargo clippy -p dstack-vmm --bins -- -D warnings, cargo fmt --check.