simpletrace: Move st_init() error reporting

User emulator builds do not have error_report() so it should not be used
by simpletrace.c.  In fact, error reporting inside simpletrace.c is
inappropriate and should be done by the caller instead.

This patch moves st_init() error reporting out to its caller,
vl.c:main().

Reported-by: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/vl.c b/vl.c
index 5e007a7..b1a94aa 100644
--- a/vl.c
+++ b/vl.c
@@ -2766,7 +2766,9 @@
     }
     loc_set_none();
 
-    st_init(trace_file);
+    if (!st_init(trace_file)) {
+        fprintf(stderr, "warning: unable to initialize simple trace backend\n");
+    }
 
     /* If no data_dir is specified then try to find it relative to the
        executable path.  */