core: Storage activation error fix

When an already active storage domain was activated through rest a response
body claiming it's inaccessible was returned. It's now telling the storage
domain's status (active).

Change-Id: Ibe8ad96a5bd40adb56b7a63c4ae5ef75a550e5b2
Bug-Url: https://bugzilla.redhat.com/849427
Signed-off-by: Vered Volansky <vvolansk@redhat.com>