KVM: SVM: Add module parameter to enable SEV-SNP

Add a module parameter than can be used to enable or disable the SEV-SNP
feature. Now that KVM contains the support for the SNP set the GHCB
hypervisor feature flag to indicate that SNP is supported.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Message-ID: <20240501085210.2213060-18-michael.roth@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>