Add XenClean and XenBootFix instructions

[dinhngtu]

These tools are already usable regardless of the driver's signing state.

I took the opportunity to add instructions for fixing the Windows boot configuration after a failed update.

Before submitting the pull request, you must agree with the following statements by checking both boxes with a 'x'.

• "I accept that my contribution is placed under the CC BY-SA 2.0 license [1]."
• "My contribution complies with the Developer Certificate of Origin [2]."

[1] https://creativecommons.org/licenses/by-sa/2.0/
[2] https://docs.xcp-ng.org/project/contributing/#developer-certificate-of-origin-dco