[root@xen xen]# virsh start Backup
libvir: Xen Daemon error : POST operation failed: (xend.err 'Device 0 (vif) could not be connected. Hotplug scripts not working.')
error: Failed to start domain Backup

[root@xen xen]# cat Backup.xml
<domain type='xen' id='-1'>
  <name>Backup</name>
  <uuid>47893b71b0f5cf98984d039ee5431ab0</uuid>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type>linux</type>
  </os>
  <memory>524288</memory>
  <currentMemory>404480</currentMemory>
  <vcpu>1</vcpu>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <interface type='bridge'>
      <source bridge='eth0'/>
      <mac address='00:16:3e:4f:f8:30'/>
      <script path='vif-bridge'/>
    </interface>
    <disk type='block' device='disk'>
      <driver name='phy'/>
      <source dev='/dev/xen/Backup_Root'/>
      <target dev='xvda'/>
    </disk>
    <graphics type='vnc' port='-1'/>
  </devices>
</domain>