virtme-run: Only add a balloon if --balloon is specified

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
virtme-run: Add --disk

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Add a sample xfstests driver.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>