Add support for GPU in Incus

[etiennedub]

Add support for GPUs in Incus by explicitly passing the PCI IDs of the GPU devices as a list.

This has multiple limitations:

1. The NVIDIA driver and the NVIDIA Container Toolkit must be installed on the Incus host.
2. It only works with unprivileged containers (there is a validation in the code for this).
3. The node count must be 1 (no validation is currently enforced).