KVM: x86: Add comments to document various emulation types

Document the intended usage of each emulation type as each exists to
handle an edge case of one kind or another and can be easily
misinterpreted at first glance.

Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>