sev: use explicit indices for mapping firmware error codes to strings

sev: use explicit indices for mapping firmware error codes to strings

This can help lower any margin for error when making future additions to
the list, especially if they're made out of order.

While doing so, make capitalization of ASID consistent with its usage in
the SEV firmware spec (Asid -> ASID).

Signed-off-by: Connor Kuehl <ckuehl@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210430134830.254741-2-ckuehl@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>