core: validate IQN on update iSCSI connection

* Add a canDo validation to UpdateStorageServerConnectionCommand
  for empty IQN (similar to AddStorageServerConnectionCommand).
* Updated unit tests accordingly.

Change-Id: I437d93f90f6199238cc7cbe9c264c5bf72cca71c
Bug-Url: https://bugzilla.redhat.com/1005284
Signed-off-by: Daniel Erez <derez@redhat.com>