libvirt: merge two utils tests files

The nova/virt/libvirt/utils.py file had some of its tests in
test_utils.py and some in test_image_utils.py. Move them all
into test_utils.py, so the test filename matches the source
filename

Change-Id: I1001e1485646fc3546e86ddb25eb2d6ad8c88315
Merge "libvirt: merge two utils tests files"