virtio-net: count VIRTIO_NET_F_MAC when calculating config_len

Commit 14f9b664 (hw/virtio-net.c: set config size using host features) tries to
calculate config size based on the host features. But it forgets the
VIRTIO_NET_F_MAC were always set for qemu later. This will lead a zero config
len for virtio-net device when both VIRTIO_NET_F_STATUS and VIRTIO_NET_F_MQ were
disabled form command line. Then qemu will crash when user tries to read the
config of virtio-net.

Fix this by counting VIRTIO_NET_F_MAC and make sure the config at least contains
the mac address.

Cc: Jesse Larrew <jlarrew@linux.vnet.ibm.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1366874814-2658-1-git-send-email-jasowang@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
