Description of problem:
As a result of regression in 4.5, copying disks from traditional (image) domains to MBS fails with:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/vdsm/storage/task.py", line 884, in _run
    return fn(*args, **kargs)
  File "</usr/lib/python3.6/site-packages/decorator.py:decorator-gen-199>", line 2, in sdm_copy_data
  File "/usr/lib/python3.6/site-packages/vdsm/common/api.py", line 50, in method
    ret = func(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/vdsm/storage/hsm.py", line 3357, in sdm_copy_data
    copy_bitmaps=copy_bitmaps)
  File "/usr/lib/python3.6/site-packages/vdsm/storage/sdm/api/copy_data.py", line 60, in __init__
    lock_image=source['img_id'] != destination['img_id'])
KeyError: 'img_id'

As img_id is not available for the external endpoint used by Managed Block Storage disks in copy_data

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


How reproducible:
100%

Steps to Reproduce:
1. Try to copy a disk to Managed Block Storage domain
2.
3.

Actual results:
Fails with the exception in the description


Expected results:
Succeed

Additional info: