igvm: igvm crate must offer mode where failure to recognize a header is not fatal

[chris-oo]

The igvm_defs specification is intended to allow older loaders to load newer files. The current igvm implementation does not do this, on seeing headers it does not recognize, it will emit a failure instead of ignoring the header.

Add a mode to support this.

[luigix25]

does this issue relate to this #121 ?

[chris-oo]

somewhat yes, but also i think we may need more permissive modes where we don't fail for even required headers (you might just get the wrong measurement though). Something we're thinking about.

[luigix25]

yep, totally agree! Every new directive added in the igvm file means breaking backwards compatibility, this is why I came up with that PR!

[chris-oo]

Yeah this is something we're also grappling with for corim. I'm hoping to have some time in July to have something for this.