Handle kernels with no ipv6 support

If the ipv6 kernel module is not loaded, then we get this when starting
a virtual network:

  libvir: Network Config error :
  cannot enable /proc/sys/net/ipv6/conf/virbr0/disable_ipv6:
  No such file or directory

If disable_ipv6 is not present, we should just merrily continue on our
way.

* src/network_driver.c: make networkDisableIPV6() not fail if the kernel
  has no ipv6 support

src/network_driver.c

@@ -801,6 +801,12 @@ static int networkDisableIPV6(virConnectPtr conn,
         goto cleanup;
     }
 
+    if (access(field, W_OK) < 0 && errno == ENOENT) {
+        VIR_DEBUG("ipv6 appears to already be disabled on %s", network->def->bridge);
+        ret = 0;
+        goto cleanup;
+    }
+
     if (virFileWriteStr(field, "1") < 0) {
         virReportSystemError(conn, errno,
                              _("cannot enable %s"), field);