trace: Multi-backend tracing

Adds support to compile QEMU with multiple tracing backends at the same time.

For example, you can compile QEMU with:

  $ ./configure --enable-trace-backends=ftrace,dtrace

Where 'ftrace' can be handy for having an in-flight record of events, and 'dtrace' can be later used to extract more information from the system.

This patch allows having both available without recompiling QEMU.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qemu-io.c

@@ -433,7 +433,7 @@ int main(int argc, char **argv)
             }
             break;
         case 'T':
-            if (!trace_backend_init(optarg, NULL)) {
+            if (!trace_init_backends(optarg, NULL)) {
                 exit(1); /* error message will have been printed */
             }
             break;