Fix blocking issue when powervm calculate checksum

In powervm driver, it takes long time to calculate file checksum
before and after transferring image file. Because nova-compute is
a single thread, it won't process any other work at the same time.
Here give other threads chance to run by using time.sleep(0), that
acts as a yield of the thread for eventlet.

Fixes bug #1181895

Change-Id: I79fbd9d7d4fb7791ca599b136cf9945dec8996e2
Merge "Fix blocking issue when powervm calculate checksum"