trace/simple: Fix unauthorized enable

trace/simple: Fix unauthorized enable

st_set_trace_file() accidentally enables tracing.  It's called
unconditionally during startup, which is why QEMU built with the
simple trace backend always writes a trace file "trace-$PID".

This has been broken for quite a while.  I didn't track down the exact
commit.

Fix st_set_trace_file() to restore the state.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200527065613.25322-1-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>