Instance migration: remove error on missing link

Since we don't support upgrades from 1.2.4 without restarting the
instance, the 'not restarted since 1.2.5' check/error is
wrong/misleading.

Since the live migration works anyway without the links (it recreates
them during the disk reconfiguration anyway), we remove the check and we
transform it into a warning (to the node daemon log only,
unfortunately).

For 2.3, we'll need to change the symlink creation from instance start
time to disk activation time (but that requires more RPC changes).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>