core: return object locked message when storage domain is locked.

This patch changed the returned canDoAction in case of a locked storage
domain to ACTION_TYPE_FAILED_OBJECT_LOCKED instead of
ACTION_TYPE_FAILED_STORAGE_DOMAIN_STATUS_ILLEGAL.

Change-Id: I209b624cd68050ebfe7eeed49b1350e37832e4aa
Bug-Url: https://bugzilla.redhat.com/879355
Signed-off-by: Liron Aravot <laravot@redhat.com>