Description of problem:
Insufficient information is displayed for failed live migration of VM with direct pass-through lun when the direct LUN is not available on the destination host.

Version-Release number of selected component (if applicable):
RHV 4.0

How reproducible:
100%

Steps to Reproduce:
1. Create VM with direct LUN pass-through 
2. Zone this LUN to source hypervisor and start the VM
3. live migrate the VM to a host where the LUN is not available

Actual results:
 2017-05-14 12:00:01,003 | Migration failed  (VM: VM01, Source: host01.example.com).
 2017-05-14 12:00:01,002 | No available host was found to migrate VM VM01 to.
 2017-05-14 12:00:01,001 | Failed to migrate VM VM01 to Host host02.example.com . Trying to migrate to another Host.

Expected results:

 Error message should let the user know whay exactly the migration failed. In this case the following information whoudl be displayed:

    1) The Direct pass through LUN xxxx-xxxx-xxxx-xxxx is not available on the host host02.example.com

The user has to be able to identify the issue without investigating the logs.

Additional info:

Destination vdsm:

Thread-1534::INFO::2017-05-14 11:53:45,842::logUtils::52::dispatcher::(wrapper) Run and protect: getDevicesVisibility, Return response: {'visible': {u'2dbb99ec54087f1816c9ce900af073231': False}}
Thread-1534::DEBUG::2017-05-14 11:53:45,842::task::1193::Storage.TaskManager.Task::(prepare) Task=`e5d8fe46-a8a8-454d-9d5f-75e0e3f58b35`::finished: {'visible': {u'2dbb99ec54087f1816c9ce900af073231': False}}

...

Thread-1534::DEBUG::2017-05-14 11:53:45,843::task::995::Storage.TaskManager.Task::(_decref) Task=`e5d8fe46-a8a8-454d-9d5f-75e0e3f58b35`::ref 0 aborting False
Thread-1534::ERROR::2017-05-14 11:53:45,843::vm::773::virt.vm::(_startUnderlyingVm) vmId=`38639856-6f6a-48ef-b848-d7a1264aa994`::The vm start process failed
Traceback (most recent call last):
  File "/usr/share/vdsm/virt/vm.py", line 714, in _startUnderlyingVm
    self._run()
  File "/usr/share/vdsm/virt/vm.py", line 1940, in _run
    self._preparePathsForDrives(dev_spec_map[hwclass.DISK])
  File "/usr/share/vdsm/virt/vm.py", line 799, in _preparePathsForDrives
    drive['path'] = self.cif.prepareVolumePath(drive, self.id)
  File "/usr/share/vdsm/clientIF.py", line 331, in prepareVolumePath
    raise vm.VolumeError(drive)
VolumeError: Bad volume specification {u'index': 0, u'iface': u'scsi', u'apparentsize': u'107374182400', u'name': u'sda', u'format': u'raw', u'bootOrder': u'1', u'sgio': u'filtered', u'GUID': u'2dbb99ec54087f1816c9ce900af073231', u'specParams': {}, u'readonly': u'False', u'alias': u'scsi0-0-0-0', u'optional': u'false', u'deviceId': u'97dc97a4-8b6a-4b32-9b71-13242c0b75ff', u'truesize': u'107374182400', u'address': {u'bus': u'0', u'controller': u'0', u'type': u'drive', u'target': u'0', u'unit': u'0'}, u'device': u'lun', u'shared': u'false', u'path': u'/dev/mapper/2dbb99ec54087f1816c9ce900af073231', u'propagateErrors': u'off', u'type': u'disk'}