allow none checksumtype for qemu builder (copy/paste from virtualbox)

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
add missing variable

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Merge pull request #1645 from vtolstov/qemu_disk

Allow none checksumtype for qemu builder