VM migration failed because path preparation timed out on the destination host due to the VM failing to start because of an erroneous floppy device definition

Issue

• A host could not be placed into maintenance mode because VMs were failing to migrate off of the host.

• Some VMs failed to migrate to another host due to the following error on the destination host;

  Thread-10854::ERROR::2015-11-13 14:41:57,728::vm::2344::vm.Vm::(_startUnderlyingVm) vmId=`5663168a-1577-418e-a269-48e748f57433`::The vm start process failed
  Traceback (most recent call last):
    File "/usr/share/vdsm/virt/vm.py", line 2284, in _startUnderlyingVm
      self._run()
    File "/usr/share/vdsm/virt/vm.py", line 3276, in _run
      devices = self.buildConfDevices()
    File "/usr/share/vdsm/virt/vm.py", line 2089, in buildConfDevices
      self.normalizeDrivesIndices(devices[DISK_DEVICES])
    File "/usr/share/vdsm/virt/vm.py", line 2252, in normalizeDrivesIndices
      if drv['iface'] not in self._usedIndices:
  KeyError: 'iface'
  

• This resulted in path preparation timing out;

  Thread-10853::DEBUG::2015-11-13 14:46:45,386::vm::5685::vm.Vm::(waitForMigrationDestinationPrepare) vmId=`5663168a-1577-418e-a269-48e748f57433`::Timeout while waiting for path preparation
  

• These VMs had been created from the same template as other VMs that were unaffected by this problem.