ux: reimplemented OS installation

Before we were asking the user to press enter when
finished in order to destroy the VM and create it
again for booting from HD.

Now the VM is always powered off so we can detect
when the VM is down and ask the user if the OS
was successfully installed and allow the user to
try again the OS installation or abort the setup.

Change-Id: I968843265fc4e914a63771c63479dce237ee6b1a
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>