installer: Setup drivers in construction method

This implies moving of drivers setup (which may involve downloading)
from wizard's setup-to-review transition to preparing stage. For media
automatically found by Boxes, drivers setup will happen even before user
sees it listed in wizard's 'source' page.

This should probably be squashed into patch:

"installer: Download & Install pre-installation drivers"

but i wanted to get feedback on this before I squash it.

https://bugzilla.gnome.org/show_bug.cgi?id=676537