Make the guest tools ISO 'optional'.

This should be 'required' as it is currently but since it's not buildable
from the OSS bits yet it should be 'optional' till that's sorted.
Merge pull request #1 from dickon/master

temporarily remove xctools from default steps since it incorrectly tries to rsync / into the build tree if not carefully configured
Merge pull request #3 from flihp/master

Make the guest tools ISO 'optional'.