Fix some bugs in reboot

There are two issues fixed in this patch:
  - first, the recent RPC changes caused loss of data in hard reboot
    type; we weren't reporting any results from the stop/start instance
    calls;
  - second, in soft or hard reboots, we didn't initialized the disk
    physical ID; based on the last state of the instance's disks, this
    can create a failure in identifying the disks

After this patch, burnin works again with reboot, and reports errors
correctly.

Reviewed-by: imsnah