rbd: fix chunk size units

A typo used XOR instead of python's operator for exponentiation in the
migration to glance_store (5e8eb74438423a4d6134af964fd522bf0ca78429).
Change it back to the correct operator.

Change-Id: I80ba27137e56b3b7aa5ef232e4d770fd363dd3c2
Closes-Bug: 1370247
Merge "rbd: fix chunk size units"