doc(XCP-ng): Add mention of Viridian extensions

[thomas-dkmt]

Following the documentation of Viridian extensions in the Xen Orchestra documentation (see this PR), this PR adds a brief mention of Viridian in the XCP-ng documentation. Additionally, a link to the Viridian section of the XO technical documentation is included for users who want to learn more.

[dinhngtu]

Instead of pointing to the XO docs, I'd prefer stating that Viridian enlightenments are enabled by default on Windows VM templates included with XCP-ng, as well as having instructions on how to enable/disable Viridian with xe.

[nathanael-h]

@dinhngtu maybe it should be interested to explain in XCP-ng doc the other platform args [1] needed for Windows VMs? Even if the base template set them. Don't you think?
cf. https://github.com/xenserver/guest-templates-json/blob/master/json/base-windows.json

[1]

        "device_id": "0002",
        "viridian": "true",
        "viridian_time_ref_count": "true",
        "viridian_reference_tsc": "true",
        "viridian_apic_assist": "true",
        "viridian_crash_ctl": "true",
        "viridian_stimer": "true",

[dinhngtu]

@dinhngtu maybe it should be interested to explain in XCP-ng doc the other platform args [1] needed for Windows VMs? Even if the base template set them. Don't you think? cf. https://github.com/xenserver/guest-templates-json/blob/master/json/base-windows.json

An explanation of device_id would probably be helpful as it's used by various things (namely xcp-ng-tests) to detect Windows VMs.

For the individual Viridian settings, IMO they're only needed for completeness. The settings correspond to the docs here https://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html#Support-for-Paravirtualisation-of-HVM-Guests (but note that many of the upstream settings are not supported in XCP-ng)

[dinhngtu]

Thanks, it looks good to me. Just a couple changes: