XCP-ng-8.1 host installer patch for showing kernel alt warning

[rushikeshjadhav]

Partial patch to address https://github.com/xcp-ng/xcp/issues/340

• [ ] modify the install so that it displays a notice about kernels if booted from install-alt (need to update the text)

[stormi]

Looks good to me. I'll modify the text as we devised and fix the indentation at the same time, then merge.

[stormi]

Merged manually, thanks!