newguest: attempt to set up vagrant.truphone.com

* reduce the amount of RAM (since I will have *many* VMs
* replace deprecated/one-liner options with their recommended equivalents
* reindent extra-args
* change preseed url to one that I control
Move the preseed file to a VM within the default network

github insists on https, but the debian installer doesn't have certs
or something, so the https get fails.
newguest: check that preeseed url is reachable

sometimes I forget to launch the vm that serves this.
use KVM and writeback cacheing

This should make everything super-fast
copy newguest to bootstrap with no changes.
modify bootstrap to install interactively to /dev/sda

FIXME: check that /dev/sda is unformatted before continuing
Make bootstrap interactive

Really, I should decide a bunch of defaults and put them in a preseed file.