core: Replace validation for Storage Domain existence

Changing the validation of Storage Domain with Storage Pool for
unregistered disks, since the Storage Domain can be without Storage
Pool.

Change-Id: I26b031709faae5234a26c864f3d80ff9e6d19f28
Bug-Url: https://bugzilla.redhat.com/954344
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>