启动default时出现如下错误提示：
[root@node1 ~]# virsh net-start default
error: Failed to start network default
error: internal error Network is already in use by interface eth0

提示信息表明，default所在的网段被eth0占用。
网上查寻结果表明，问题出在eth0的配置上。
检查eth0配置，发现之前添加的eth0:0网段与default网段冲突。删除eth0:0配置文件，重启网络，再次启动default，成功。

" I finally figured it out. The solution was actually quite simple. In virt-manager when you setup a connection it asks for the network address. The address I had previously setup for eth0 was xxx.xxx.xxx.64/26. I used this same one for the virt-manager connection. Obviously, this is why I was getting the error "Error starting network: Internal error Network is already in use by interface eth0".

To fix it, in virt-manager i just used xxx.xxx.xxx.64/27, giving the
connection a different block of addresses. This solved the problem
entirely. I can't believe it was that simple."
(参考: http://ubuntuforums.org/showthread.php?t=1789126)