Why live migrating VMs are failing with error "NoneType' object has no attribute 'attrib'"

Issue

• Live migrating VMs are always failing with error below in the vdsm log.

2018-07-19 18:59:24,049+0530 ERROR (vm/d44e901a) [virt.vm] (vmId='d44e901a-6280-439a-b983-1212ff6a9b1f') The vm start process failed (vm:943)
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vm.py", line 872, in _startUnderlyingVm
    self._run()
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vm.py", line 2777, in _run
    self._devices = self._make_devices()
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vm.py", line 2626, in _make_devices
    disk_objs = self._perform_host_local_adjustment()
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vm.py", line 2674, in _perform_host_local_adjustment
    self.id, self._domain, self._md_desc, self.log)
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vmdevices/common.py", line 317, in storage_device_params_from_domain_xml
    params.append(storagexml.parse(dev_elem, dev_meta))
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vmdevices/storagexml.py", line 80, in parse
    params, disk_type, vmxml.find_first(dev, 'source', None)
  File "/usr/lib/python2.7/site-packages/vdsm/virt/vmdevices/storagexml.py", line 129, in _update_source_params
    path = source.attrib.get('file', '')
AttributeError: 'NoneType' object has no attribute 'attrib'