[Libguestfs] [PATCH v2] v2v: don't fail when virtio-win does not have qemu-ga

This is my version of this patch which I think improves it in a number of ways. Firstly instead of having the bare boolean parameter ‘ok_if_missing’ we pass in the function we want to call along the directory missing path. This change then allows us to print a more useful error or warning message given the context of the call, and the new message is actionable too, so the user knows what has to be done to correct the problem. The other changes are all whitespace. This is only compile tested, please see if it fixes the problem in the slow tests. Rich.