Test suite: Download images for VERIFY.

This largely removes the need to run PREPARE, which should make
running the test suite significantly easier.

* New vm-upload and vm-download utilities.
* The meat of VERIFY has been moved to test/testsuite-prepare and now
  calls vm-download.
* PREPARE has been renamed to test/testsuite-create-images, and now
  calls vm-upload.  (Normal people don't need to run this, and it is
  not well tested yet.)