virtio-net: don't update mac_table in error state

virtio-net: don't update mac_table in error state

mac_table was always cleaned up first in handling
VIRTIO_NET_CTRL_MAC_TABLE_SET command, and we din't recover
mac_table content in error state, it's not correct.

This patch makes all the changes in temporal variables,
only update the real mac_table if everything is ok.
We won't change mac_table in error state, so rxfilter
notification isn't needed.

This patch also fixed same problame in
 http://lists.nongnu.org/archive/html/qemu-devel/2013-11/msg01188.html
 (not merge)

I will send patch for virtio spec to clarifying this change.

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Vlad Yasevich <vyasevic@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>