[Feature Request] PVH boot support

[cperciva]

Feature Request

It would be nice to support the PVH direct boot API. Guest operating systems which have support for Xen/PVH will find this much easier than trying to support the "linux" boot API currently offered by Firecracker.

Describe the desired solution

I'd like to add support for PVH boot. Work was done (#1818) two years ago but abandoned; I'd like to get it finished.

Describe possible alternatives

There's no real alternatives for my use case of "wanting to boot FreeBSD on Firecracker".

Additional context

Working with the branch from two years ago, I currently have FreeBSD reaching MI kernel startup and expect more progress soon.

Checks

• Have you searched the Firecracker Issues database for similar requests?
• Have you read all the existing relevant Firecracker documentation?
• Have you read and understood Firecracker's core tenets?