Shut down the appliance cleanly

Shut down the appliance cleanly

When guestfsd exits, or the user exits the virt-rescue shell, the init script
exits which causes the kernel to panic. This isn't really a functional issue, as
all useful work is done by this point. However, it does cause virt-rescue to
display an unsightly error message.

This patch causes the appliance to power off cleanly before the init script
exits. Note it actually does a reboot rather than a poweroff. This is because
ACPI is disabled in the appliance, meaning poweroff doesn't work, but qemu is
configured not to restart on reboot.