Implement support for multi devices changes

This big patch adds support for:
  - changing NIC/disks in the multi-device model
  - adding/removing NICs
  - adding/removing disks

The patch is big and not very nice; the error checking paths are not
very clear.

The biggest problem is that from a simple instance.ATTR=VAL change
(which didn't throw errors before) now we are creating and removing
disks in this LU.

Reviewed-by: imsnah