unattended-installer: Extract boot files before injection
unattended-installer: Extract boot files before injection

This moves the extraction of the boot files to be before the injection
of the automated installation scripts. This is necessary since in some
cases (e.g. Debian-based OSes) these scripts will be placed within
these boot files.

https://bugzilla.gnome.org/show_bug.cgi?id=730640

vbenes/gnome-boxes,vbenes/gnome-boxes