Make NICInfo string fields non-const

We now only assign strdup()ed strings to these fields, never static
strings.

aliguori: fix build for ppc_prep and mips_jazz

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/net.c b/net.c
index f2b954c..76e44c9 100644
--- a/net.c
+++ b/net.c
@@ -2813,10 +2813,10 @@
     nd->vlan->nb_guest_devs--;
     nb_nics--;
 
-    qemu_free((void *)nd->model);
-    qemu_free((void *)nd->name);
-    qemu_free((void *)nd->devaddr);
-    qemu_free((void *)nd->id);
+    qemu_free(nd->model);
+    qemu_free(nd->name);
+    qemu_free(nd->devaddr);
+    qemu_free(nd->id);
 
     memset(nd, 0, sizeof(*nd));
 }