qemu: Don't report error on successful media eject

qemu: Don't report error on successful media eject

If we are just ejecting media, ret == -1 even after the retry loop
determines that the tray is open, as requested. This means media
disconnect always report's error.

Fix it, and fix some other mini issues:

- Don't overwrite the 'eject' error message if the retry loop fails
- Move the retries decrement inside the loop, otherwise the final loop
  might succeed, yet retries == 0 and we will raise error
- Setting ret = -1 in the disk->src check is unneeded
- Fix comment typos

cc: mprivozn@redhat.com