hw/net/xen_nic.c: Set 'netdev->mac' to NULL after free it

Since net_init() checks whether 'netdev->mac' is NULL, before alloc it;
net_release() also need set 'netdev->mac' to NULL after free it.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
