manager: return abstract base type

Unit-tests later required knowing the vtable for a BlobManager, and this
should avoid that needless complication.

Change-Id: I7a208d08c2065b7383fd5095e00d17c1ab3b0cf5
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/blobs-ipmid/manager.hpp b/blobs-ipmid/manager.hpp
index 14c8b98..884271c 100644
--- a/blobs-ipmid/manager.hpp
+++ b/blobs-ipmid/manager.hpp
@@ -256,6 +256,6 @@
  *
  * @return a pointer to the BlobManager instance.
  */
-BlobManager* getBlobManager();
+ManagerInterface* getBlobManager();
 
 } // namespace blobs