Generalize vGPU device lifecycle

[yummybomb]

summary

• represent vGPU attachments with a framework-neutral device path while retaining the mdev UUID for API compatibility
• route instance create, start, stop, delete, and QEMU configuration through generic vGPU helpers
• preserve metadata written before the generic device fields were added
• keep runtime behavior mdev-only in this PR

This is the behavior-preserving base for vendor-specific VFIO support.

tests

• go test -race ./lib/devices ./lib/hypervisor/qemu ./lib/instances -run 'BuildArgs_VGPU|StoredVGPU' -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