remove intermediate libvirt downloaded images

* nova/virt/images.py (fetch_to_raw): When converting a downloaded
image to raw, ensure we delete the original download image.
I.E. the .part file.  The regression was introduced in 7c265343.
* nova/tests/test_libvirt.py (test_fetch_raw_image): A new test
for this function which wasn't tested previously.

Fixes bug: 1125068
Change-Id: Ie6f2cff1a554b8500fd66f0832566ae1b317be40
(cherry picked from commit 1501a4e4f75efb63385a1dadc728c2ef245bf7de)