qemu-dbus: prevent double start of the bus daemon

Allow calling qemuDBusStart() multiple times (as may be done by
qemu-slirp already).
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_dbus.c

@@ -177,6 +177,9 @@ qemuDBusStart(virQEMUDriverPtr driver,
     pid_t cpid = -1;
     int ret = -1;
 
+    if (priv->dbusDaemonRunning)
+        return 0;
+
     if (!virFileIsExecutable(cfg->dbusDaemonName)) {
         virReportSystemError(errno,
                              _("'%s' is not a suitable dbus-daemon"),