Error: Device 0 (vif) could not be connected. Could not find bridge device xenbr0
问题说明：centos下xenbr0未启动，导致xen中虚拟机不能正常启动。
解决办法：运行一下命令:
#brctl show //查看xen网桥是否启动
#/etc/xen/scripts/network-bridge start //启动网桥