trace: convert stderr backend to log

[Also update .travis.yml --enable-trace-backends=stderr
--Stefan]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1452174932-28657-10-git-send-email-den@openvz.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/vl.c b/vl.c
index 58cc050..a7d7c39 100644
--- a/vl.c
+++ b/vl.c
@@ -4115,6 +4115,8 @@
             exit(1);
         }
         qemu_set_log(mask);
+    } else {
+        qemu_set_log(0);
     }
 
     if (!trace_init_backends()) {