Add new reboot_time test case

Besides the boot time of a guest (boot_time), we would like to measure
the reboot time. This TC does essentially the same as boot_time, but
will instead issue the 'reboot' command to the VM, and measure the time
until the console is again available for login.

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>
New TC: Save and restore VM to/from file

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>