KVM: SEV: Disable SEV-SNP support on initialization failure

During platform init, SNP initialization may fail for several reasons,
such as firmware command failures and incompatible versions. However,
the KVM capability may continue to advertise support for it.

The platform may have SNP enabled but if SNP_INIT fails then SNP is
not supported by KVM.

During KVM module initialization query the SNP platform status to obtain
the SNP initialization state and use it as an additional condition to
determine support for SEV-SNP.

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Pratik R. Sampat <prsampat@amd.com>
Signed-off-by: Pratik R. Sampat <prsampat@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Pavan Kumar Paluri <papaluri@amd.com>
Message-ID: <20250512221634.12045-1-Ashish.Kalra@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>