core: introduce GetUnregisteredBlockStorageDomains query

Introduced a new command for discovering unregistered
block (iSCSI/FCP) storage domains (of type Data).

The query should get as parameters the host ID, StorageType (iSCSI/FCP)
and a list of StorageServerConnections (applicable only for iSCSI).

Finding unregistered domains flow is as follows:
* Connect into the specified targets (using ConnectStorageToVds).
* Retrieve associated LUNs (using GetDeviceList).
* Filter irrelevant LUNs by crossing with the specified targets and
  LUNs that are already a part of registered storage domains.
* Fetching volume group IDs from the LUNs list.
* For each VG-ID, use GetVGInfo to get VG-Name (storage domains ID).
* Invoke GetStorageDomainInfo on each storage domain ID
  and build the StorageDomain objects.
* Finally, return to the client a list of storage domains and
  successfully logged-in connections.

Change-Id: I71cad5562f8d7dcb39c5a99b7daea28ccb4a00cf
Signed-off-by: Daniel Erez <derez@redhat.com>