KVM: SEV: Update KVM_AMD_SEV Kconfig entry and mention SEV-SNP

KVM: SEV: Update KVM_AMD_SEV Kconfig entry and mention SEV-SNP

SEV-SNP support is present since commit 1dfe571c12cf ("KVM: SEV: Add
initial SEV-SNP support") but Kconfig entry wasn't updated and still
mentions SEV and SEV-ES only. Add SEV-SNP there and, while on it, expand
'SEV' in the description as 'Encrypted VMs' is not what 'SEV' stands for.

No functional change.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240828122111.160273-1-vkuznets@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>