Merge branch 'devel-2.7'

* devel-2.7
  cfgupgrade: Fix error messages
  Update "FIXME" string in RAPI
  rapi client: add target_node to migrate instance
  Make diskless instances externally mirrored
  Fix migrate/failover -n for ext mirror storage

Trivial merge: no conflicts

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>