Implement all hv functions in hv_chroot/hv_fake

The chroot and fake hypervisors were missing:

- the powercycle node functionality
- proper handling of migration requests

The powercycle was just used as in the other hypervisors (use the
standard linux powercycle). The migration for chroot was disabled
explicitly, whereas for the fake one it was implemented to simulate
correctly. This required some work on the fake hypervisor, but now the
implementation of start/stop/etc. is much more clean.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>