openhcl_boot/cvms: support enlightened panic

[chris-oo]

The hypervisor supports this, so we should enable enlightened panic support and report something useful.

Note that this requires a few things:

1. minimal_rt needs to support a different method of writing to the crash MSRs (trait based?) since on a CVM we cannot use wrmsr, instead they must be GHCB calls/etc
2. the message page must be host visible if we want to send a message. If we do not want to do that, we need to think of some other way to encode panic info. One idea is to maybe make a page host visible only in the crash path, and if that fails just continue reporting without it.