vm-creator: Don't ignore force shutdown

Instead of completing ignoring force shutdown and not checking for
installations at all, just don't automatically delete live VMs on force
shutdown when no installation is performed by last live session.

Some OSes (e.g Ubuntu) require you to eject the CDROM before rebooting
after installation. Since we don't allow that during installation/live
session (see commit 4bed1f5 or bug#690779 for details on why), only
option user has is to work around this by force shutting down the VM and
that won't actually work w/o this change as VM will continue to be
marked as 'live'.

https://bugzilla.gnome.org/show_bug.cgi?id=707803