v2v: Don't use Unix.unlink since Unix module is open in this file.

Minor code refactor.
v2v: Add --debug-overlays option.

You can use this to accelerate testing:

  $ virt-v2v --no-copy --debug-overlays [etc]
  [...]
  [  82.0] Converting Windows 7 Ultimate to run on KVM
  This guest has virtio drivers installed.
  [  86.0] Closing the overlay
  [  91.0] Creating output metadata
  Overlay saved as /var/tmp/win7-test-sda.qcow2 [--debug-overlays]
  [  91.0] Finishing off

No disk format conversion/copying is done, so it's very fast.

The overlay file is preserved and can be tested directly (it uses the
source file as backing).
v2v: windows: Sort files before uploading.

This makes it easier to compare the trace output from different runs
of virt-v2v.