vl.c: Guard against GThread double-initialization

GThread-ERROR **: GThread system may only be initialized once.
aborting...

Making the g_thread_init() call in vl.c conditional resolves an abort on
Mac OS X, where coroutine-gthread.c seems to call it before vl.c.
Reported-by: NJuan Pineda <juan@logician.com>
Signed-off-by: NAndreas Färber <andreas.faerber@web.de>
Cc: Daniel P. Berrange <berrange@redhat.com>

vl.c

@@ -2185,7 +2185,9 @@ int main(int argc, char **argv, char **envp)
     error_set_progname(argv[0]);
 
     g_mem_set_vtable(&mem_trace);
-    g_thread_init(NULL);
+    if (!g_thread_supported()) {
+        g_thread_init(NULL);
+    }
 
     runstate_init();