core: Fix NPE when scanning for non-existent domain.

Adding a validation in GetUnregisteredDiskQuery for existence of storage domain
by the storage pool id and storage domain id, transferred in the parameters.

Change-Id: I7c722ca3b003efd89ae5207bc941d6926b5f052c
Bug-Url: https://bugzilla.redhat.com/954344
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>