This tests the 3 main operations in removeable media

 - Eject existing media
 - Insert new media
 - Change existing media (eject followed by insert)

* lib/Sys/Virt/TCK.pm: Allow a specific ostype to be requested
  when getting a guest config
* scripts/domain/207-disk-media-change.t: Test media change,
  eject, and re-insert