Support vendor VFIO vGPU devices

[yummybomb]

summary

• detect NVIDIA's mdev and vendor-specific VFIO frameworks at runtime
• discover vendor VFIO virtual functions and parse creatable_vgpu_types
• assign and release profiles through current_vgpu_type with least-loaded-GPU placement
• preserve active assignments and clear orphaned assignments by checking vendor VFIO device handles
• document both frameworks and verify that vendor VFIO vGPUs retain the existing standby rejection

Stacked on #322, which contains the framework-neutral lifecycle refactor. Live host validation will follow after host provisioning supports vendor-specific VFIO.

tests

• go test -race ./lib/devices ./lib/instances -run 'VendorVFIO|CreatableVGPU|StoredVGPU|StandbyRejectsVendorVFIO' -count=1
• go test ./lib/devices ./lib/hypervisor/qemu ./lib/resources -count=1
• go test ./cmd/api ./cmd/api/api ./lib/instances ./integration -run '^$' -count=1
• go vet ./cmd/api ./lib/devices ./lib/hypervisor/qemu ./lib/instances ./lib/resources