Add Device interface for testing

The NVML driver logic has grown more complex as users have started using this driver for fingerprinting/reserving various types of Nvidia devices. It's reasonable to assume this logic should have basic unit tests. However, in it's current form, its quite difficult to test this logic. Only the `nvmlClient` is testable.

We should refactor the driver logic to make it easier for our community members to unit test their contributions.

This is up for debate but one way to do this would be to move away from using the package function where possible, and use `nvml.DeviceGetHandleByUUID` to get a handle to the device, which is an interface that can be mocked.

Start by tracing the NVML driver logic around the currently testable nvmlClient and the package functions used for device lookup. Evaluate introducing a mockable Device interface around nvml.DeviceGetHandleByUUID, then add unit tests showing that fingerprinting and device-reservation logic can run without real NVML hardware.

Written by the indexing model from the issue text.