This makes it possible to lock the log file; it also makes log_mask_ln!
not allocate memory when logging a constant string.
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>