RHV: With a VM running (Live Merge) snapshot delete fails

Issue

• With a VM running (Live Merge) snapshot delete fails with the following error message:

Failed to delete snapshot 'Snapshot for backup script' for VM '  'EXAMPLE-VM'.

• The host VDSM logs reports the following error:

2018-01-13 14:00:19,082-0200 ERROR (jsonrpc/6) [virt.vm] (vmId='5759c22c-3114-4a06-8277-a9982591c4bf') Live merge failed (job: ccc6e0e0-6ce9-40d7-bc0a-07957fdf73f5) (vm:4803)
Traceback (most recent call last):
  File "/usr/share/vdsm/virt/vm.py", line 4799, in merge
    flags)
  File "/usr/lib/python2.7/site-packages/vdsm/virt/virdomain.py", line 69, in f
    ret = attr(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/libvirtconnection.py", line 123, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/utils.py", line 941, in wrapper
    return func(inst, *args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 674, in blockCommit
    if ret == -1: raise libvirtError ('virDomainBlockCommit() failed', dom=self)
libvirtError: invalid argument: could not find image '/rhev/data-center/mnt/blockSD/c4ee73dd-011a-4f62-9242-6f2b38499d37/images/57ac33eb-b933-4f45-a7ea-ae46b3e97dee/77213a5e-527c-4888-a20c-53a018882909' in chain for '/rhev/data-center/591093ea-0394-01f8-00ac-0000000002d0/c4ee73dd-011a-4f62-9242-6f2b38499d37/images/57ac33eb-b933-4f45-a7ea-ae46b3e97dee/77213a5e-527c-4888-a20c-53a018882909'