trace: add syslog tracing backend

This patch adds a tracing backend which sends output using syslog().
The syslog backend is limited to POSIX compliant systems.

openlog() is called with facility set to LOG_DAEMON, with the LOG_PID
option. Trace events are logged at level LOG_INFO.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Message-id: 1470318254-29989-1-git-send-email-paul.durrant@citrix.com
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

trace/control.c

@@ -19,6 +19,9 @@
 #ifdef CONFIG_TRACE_LOG
 #include "qemu/log.h"
 #endif
+#ifdef CONFIG_TRACE_SYSLOG
+#include <syslog.h>
+#endif
 #include "qapi/error.h"
 #include "qemu/error-report.h"
 #include "qemu/config-file.h"
@@ -250,6 +253,10 @@ bool trace_init_backends(void)
     }
 #endif
 
+#ifdef CONFIG_TRACE_SYSLOG
+    openlog(NULL, LOG_PID, LOG_DAEMON);
+#endif
+
     return true;
 }