KVM: SVM: Rewrite sev_es_prepare_switch_to_guest()'s comment about swap types

Rewrite the comment(s) in sev_es_prepare_switch_to_guest() to explain the
swap types employed by the CPU for SEV-ES guests, i.e. to explain why KVM
needs to save a seemingly random subset of host state, and to provide a
decoder for the APM's Type-A/B/C terminology.

Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Link: https://lore.kernel.org/r/20230615063757.3039121-4-aik@amd.com


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