Add a rpc call for BlockDev.Close()

This patch adds rpc layer calls (in rpc.py and the equivalent in
ganeti-noded) to close a list of block devices, and the wrapper in
backend.py that takes a list of Disk objects, identifies them and
returns correctly formatted results.

The reason why this very basic call was missing until now from the rpc
layer is that we usually don't care about device closes (though we
should, and will do so in the future) as only drbd has a meaningful
Close() operation; right now we directly do Shutdown().

The patch is clean enough that it's actually independent of the live
migration implementation.

Reviewed-by: imsnah