Add VM cache scanning script

[dinhngtu]

Stale cached VMs will fill up the SR and cause test failures when importing new images.

This script helps detect those. It's non-destructive; deleting the VMs is left to the user.

[glehmann]

This is not in our current dependency. Could it be added in pyproject.toml?

[dinhngtu]

Added XenAPI to dev dependencies

[glehmann]

I didn't realize that scan_cache.py was meant to be used on an xcp-ng host, not on the dev computer. The XenAPI dependency doesn't make sense in that case: I don't think we plan on cloning xcp-ng-tests on each host.

Apparently, it also requires copying vm_data.py on the host.

I think I would prefer a script that runs locally, on the dev host, and connects to the host.
Could you explain why you preferred a script to copy on the host?

[dinhngtu]

I didn't realize that scan_cache.py was meant to be used on an xcp-ng host, not on the dev computer. The XenAPI dependency doesn't make sense in that case: I don't think we plan on cloning xcp-ng-tests on each host.

Apparently, it also requires copying vm_data.py on the host.

I think I would prefer a script that runs locally, on the dev host, and connects to the host. Could you explain why you preferred a script to copy on the host?

It's better to run it locally on the dev machine rather than on the hosts. I'll make it do that.