add an init function parameter to qemu_chr_open()

And use it for the malta emulation. Fix segfault introduced in
revision 6352.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6365 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/hw/usb-serial.c b/hw/usb-serial.c
index a6a756d..9dd2c07 100644
--- a/hw/usb-serial.c
+++ b/hw/usb-serial.c
@@ -558,7 +558,7 @@
         return NULL;
 
     snprintf(label, sizeof(label), "usbserial%d", index++);
-    cdrv = qemu_chr_open(label, filename);
+    cdrv = qemu_chr_open(label, filename, NULL);
     if (!cdrv)
         goto fail;
     s->cs = cdrv;