config: Add check for disk's “iv_name”

This check verifies “iv_name” of all instance disks. If one is wrong
(which shouldn't happen in the first place), cluster verification will
warn:

“ERROR: cluster: Instance 'inst.example.com' has wrongly named disks:
name of disk 1 should be 'disk/1', but is 'disk/4'”

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