docs: update manpage for stderr->log rename

With commit ed7f5f1d8db06fc31352a5ef4f54985e630c575a the name of
this backend changed from “stderr” to “log”.

Signed-off-by: Philipp Gesang <philipp.gesang@intra2net.com>
Message-id: 20170202114101.2655-1-philipp.gesang@intra2net.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qemu-option-trace.texi

@@ -5,7 +5,7 @@ Specify tracing options.
 Immediately enable events matching @var{pattern}.
 The file must contain one event name (as listed in the @file{trace-events-all}
 file) per line; globbing patterns are accepted too.  This option is only
-available if QEMU has been compiled with the @var{simple}, @var{stderr}
+available if QEMU has been compiled with the @var{simple}, @var{log}
 or @var{ftrace} tracing backend.  To specify multiple events or patterns,
 specify the @option{-trace} option multiple times.
 
@@ -15,7 +15,7 @@ Use @code{-trace help} to print a list of names of trace points.
 Immediately enable events listed in @var{file}.
 The file must contain one event name (as listed in the @file{trace-events-all}
 file) per line; globbing patterns are accepted too.  This option is only
-available if QEMU has been compiled with the @var{simple}, @var{stderr} or
+available if QEMU has been compiled with the @var{simple}, @var{log} or
 @var{ftrace} tracing backend.
 
 @item file=@var{file}