Only declare sev_enabled() and sev_es_enabled() when CONFIG_SEV is
set, to allow the compiler to elide unused code. Remove unnecessary
stubs.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20211007161716.453984-17-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>