Add iallocator and target_node slots to OpMigrateInstance and
OpFailoverInstance to facilitate shared-storage-backed instance mobility. Add
iallocator slot to OpMigrateNode (no explicit target_node in this case).

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>