trace: switch default backend to "log"

This enables integration with other QEMU logging facilities.
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: 1452174932-28657-11-git-send-email-den@openvz.org
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

configure

@@ -303,7 +303,7 @@ pkgversion=""
 pie=""
 zero_malloc=""
 qom_cast_debug="yes"
-trace_backends="nop"
+trace_backends="log"
 trace_file="trace"
 spice=""
 rbd=""