trace: fix documentation

Mention the ftrace backend too.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1452174932-28657-2-git-send-email-den@openvz.org
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qemu-options.hx

@@ -3558,13 +3558,13 @@ Specify tracing options.
 @table @option
 @item events=@var{file}
 Immediately enable events listed in @var{file}.
-The file must contain one event name (as listed in the @var{trace-events} file)
-per line.
-This option is only available if QEMU has been compiled with
-either @var{simple} or @var{stderr} tracing backend.
+The file must contain one event name (as listed in the @file{trace-events} 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
+@var{ftrace} tracing backend.
+
 @item file=@var{file}
 Log output traces to @var{file}.
-
 This option is only available if QEMU has been compiled with
 the @var{simple} tracing backend.
 @end table