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/main.cpp b/main.cpp
index f897fda..c6b5b04 100644
--- a/main.cpp
+++ b/main.cpp
@@ -55,9 +55,8 @@
         return IPMI_CC_INVALID;
     }
 
-    BlobManager* manager = getBlobManager();
-    return processBlobCommand(command, manager, &crc, reqBuf, replyCmdBuf,
-                              dataLen);
+    return processBlobCommand(command, getBlobManager(), &crc, reqBuf,
+                              replyCmdBuf, dataLen);
 }
 
 /* TODO: this should come from the makefile or recipe... */