Forward port the live migration from 1.2 branch

This is forward port via copy (and not individual patches cherry-pick)
of the latest code on the 1.2 branch related to the migration.

The changes compared to 1.2 are the fact that we don't need the
IdentifyDisks step anymore (the drbd rpc calls are independent now), and
the rpc module improvements.

Reviewed-by: ultrotter