core: Remove StorageDomainOwnerType

The StorageDomainOwnerType enum contains only one value ("Unknown"), and
as such, is pretty useless and can be removed.

This patch removes it and all its references.

Change-Id: Iae13123bc33c2fdff2d7dd5e128c1a5cee7d262f
Signed-off-by: Allon Mureinik <amureini@redhat.com>