Factorize setting up Tails with persistence in a single step.

Factorize setting up Tails with persistence in a single step.

DRY: more and more scenarios need this kind of setup.

These steps take a long time to run, though, so at some point we'll likely need
to have a way to do this setup once only, backup the resulting disk image, and
restore it every time we need it.

Note that we make the install+persistence setup step reset the VM before each
boot. Otherwise, sometimes the VM can't start with interesting error messages.