powervm: actually remove files after migration

The powervm driver is attempting to clean up compressed image gz files
in the migration flow but the rm command being issued doesn't use the
--force option, so the files are piling up on the backing hypervisor.

This patch properly removes the files like in other parts of the powervm
code (see blockdev.py for example) and fixes the tests so the
_remove_file method is actually tested.

Closes-Bug: #1214985

Change-Id: I82919aa20447230e317c13f51bdf1c204a2eca39
Merge "powervm: actually remove files after migration"