x86/sev: Indicate the SEV-SNP guest supports Secure AVIC

Now that Secure AVIC support is complete, make it part of to the SNP present
features.

Co-developed-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Signed-off-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Link: https://lore.kernel.org/20250828113225.209174-1-Neeraj.Upadhyay@amd.com