govc: add GPU commands

[benash]

Description

This PR adds GPU management commands to the govc CLI tool, including:

• govc gpu.host.info: Display GPU information for a host, including physical NVIDIA GPUs
• govc gpu.host.profile.ls: List available vGPU profiles on a host
• govc gpu.vm.add: Add vGPU to VM with specified profile
• govc gpu.vm.info: Display GPU information for a VM
• govc gpu.vm.remove: Remove all vGPUs from VM

Commands support standard and JSON formatting.

Closes: #3689

How Has This Been Tested?

• We've done ad-hoc testing with NVIDIA A40 and L40 GPUs.
• We've asked for guidance about modifying vcsim, so that it can be properly unit tested.

Guidelines

Please read and follow the CONTRIBUTION guidelines of this project.

[dougm]

Awesome, thanks @benash ! Will review this soon.
Regarding testing, with vcsim we have a Record + Playback feature. But, we have yet to make use of that within the govmomi repo.
The default vcsim templates are captured using govc collect, see the HostHardwareInfo for example. We could probably add something so the host has a GPU by default in vcsim, I'll try to take a look at that.

[dougm]

Thanks again @benash , I pushed a small change to your branch:

• fixed up commit message (s/vcgo/govc/)
• small fix to gpu.vm.info (if vm == nil)
• add gpu.bats (we can add more at some point with vcsim support)