Replace additional os.minor by our custom version

...thus avoiding the bug in the python library.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>

lib/storage/extstorage.py

@@ -154,7 +154,7 @@ class ExtStorageDevice(base.BlockDev):
       return False
 
     self.major = os.major(st.st_rdev)
-    self.minor = os.minor(st.st_rdev)
+    self.minor = utils.osminor(st.st_rdev)
     self.attached = True
 
     return True