abi: support Siena product line

[msanft]

This adds support for the "Siena" product line, which, as per the VCEK and KDS interface spec (rev 1.0), is widely identical to the Genoa product line. Furthermore, it aligns the parsing of the model line with the specification by only referring to the extended model ID for deriving the product name from the CPUID return value, which is what Table 4 of the aforementioned specification suggests.

---

This has been tested on a Siena machine with the following example program:

package main

import (
	"fmt"

	"github.com/google/go-sev-guest/abi"
)

func main() {
	fmt.Printf("Product: %+v\n", abi.SevProduct())
}

Before this patch, this would return:

ubuntu@em-pm-testing-1:~$ ./pre-patch
Product: machine_stepping:{}

After the patch, it correctly selects the "Genoa" product line:

ubuntu@em-pm-testing-1:~$ ./post-patch
Product: name:SEV_PRODUCT_GENOA  machine_stepping:{value:2}

[deeglaze]

The Siena model in the zen4 family should still have the Siena product name.

[deeglaze]

Checking back in about this.

[msanft]

Sorry, unfortunately, I don't have access to the machine I tested this on (a Siena one) anymore. I'll see what I can do - the change you've mentioned should not be a problem at least :P

[deeglaze]

I don't think we should try to claim support without running on hardware. I also don't have access to a siena machine.

[msanft]

Ok, would propose for closing this for now then.