Re-create instance disk symlinks on activate

This patch implements recreation of instance disk symlinks when the
activate-disks operation is run. Until now, it was not possible to
re-create these symlinks without stopping and starting or migrating an
instance as the RPC call where this is done was in instance startup
and migration.

In order to do this, the blockdev_assemble rpc call needs the disk
index too, which is added to the protocol. This is a change from 2.3
and makes instance startup incompatible (FYI).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>