Annotate disk params on instance_start

We call _GatherAndLinkBlockDevs during the process, which in turn
calls _RecursiveFindBD. This needs disk parameters to work.

See also commit b8291e00.

This was reported by Ansgar and Damien.

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>