core: Use GetExistingStorageDomainList to fetch all type of Storages

Use GetExistingStorageDomainList to fetch different tpye of storage
type, so user can manipulate the Storage Domain type while importing it.

Change-Id: If3a082e0bddaad745788b9b166f2b17eff0fbd17
Bug-Url: https://bugzilla.redhat.com/1157240
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>