VM cannot start and seeing "libvirtError: internal error ifname "vnet20" not in key map" in vdsm.log

Issue

• When trying to start a VM on a specific hypervisor, it failed with the following error:

Thread-314::ERROR::2013-04-22 08:25:15,770::vm::680::vm.Vm::(_startUnderlyingVm) vmId=`cb5e3ac6-f351-4708-9729-c5287f991783`::The vm start process failed
Traceback (most recent call last):
  File "/usr/share/vdsm/vm.py", line 642, in _startUnderlyingVm
    self._run()
  File "/usr/share/vdsm/libvirtvm.py", line 1475, in _run
    self._connection.createXML(domxml, flags),
  File "/usr/lib64/python2.6/site-packages/vdsm/libvirtconnection.py", line 83, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 2645, in createXML
    if ret is None:raise libvirtError('virDomainCreateXML() failed', conn=self)
libvirtError: internal error ifname "vnet20" not in key map

• Unable to start any VMs on any hosts in the cluster.