core: rename StoragePool#storagePoolType

This patch makes simple refacroting related to
StoragePool#storagePoolType field:

- Rename it to storageType as it is obvious it is related to the pool by
  being a field of StoragePool class
- Change its type from int to Enum
- Rename its setter & getter to conform to the Java convention

Change-Id: I300741441ac76e5db3bee036ece2796383f58832
Signed-off-by: Arik Hadas <ahadas@redhat.com>