s390: Fix buggy assignment

nd->model keeps dynamically allocated model names.
So casting of a constant string is wrong here.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit 5a2b3fc5aa)

hw/s390-virtio.c

@@ -209,7 +209,7 @@ static void s390_init(ram_addr_t ram_size,
         DeviceState *dev;
 
         if (!nd->model) {
-            nd->model = (char*)"virtio";
+            nd->model = qemu_strdup("virtio");
         }
 
         if (strcmp(nd->model, "virtio")) {