Hyper-V support for IGVM optional parameter

Hi Jon,

I'm currently working on adding the device tree support for SVSM, and
I was thinking of
adding the DT directive, marking it as optional [1], for every hypervisor.

Do you know if Hyper-V supports this? QEMU currently does not, but I'm
working on it.[2]

This would simplify the logic in igvmbuilder, and when the other
hypervisors support
this directive, it would be totally transparent and require no change to SVSM.

Luigi

[1] https://docs.rs/igvm_defs/0.4.0/igvm_defs/struct.IgvmVariableHeaderType.html
[2]https://lore.kernel.org/qemu-devel/[email protected]/