x86/cpufeatures: Add "Allowed SEV Features" Feature

Add CPU feature detection for "Allowed SEV Features" to allow the
Hypervisor to enforce that SEV-ES and SEV-SNP guest VMs cannot
enable features (via SEV_FEATURES) that the Hypervisor does not
support or wish to be enabled.

Signed-off-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Link: https://lore.kernel.org/r/20250310201603.1217954-2-kim.phillips@amd.com


Signed-off-by: Sean Christopherson <seanjc@google.com>