Update blockdev's "info" at instance rename

Currently, we set "info" metadata on block devices at device creation
time, but we never update it, leading to stale data in case of
instance renames. This would not be a big problem in case of regular
renames (assuming this is a rare operation), but importing instances
into the cluster via the import/export feature usually is done with a
rename; this means that all imported instances have wrong information
in their block devices.

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