Upgrade wrong iv_names at cfgupgrade time

Since Ganeti 2.6 does warn about disks with wrong iv_names, it would
be better to fix them when upgrading to 2.6.

Not sure if this is too brittle and whether we want to add more
safeguards on the contents of the config/instance/disks before
touching them.

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