Disabling and reenabling ponsim after a Voltha restart fails

There was a change to the device_type introduced by xPON where the device_type uses the vendor ID. This change was made in PONSIM ONU. When a Voltha instance is restarted with existing PONSIM OLT and ONUs then disable and re-enable a ponsim onu fails. This is because the device_type is used to differentiate between an ONU and OLT during the reconciliation process. With the change to vendor ID then this differentiation is not possible, hence causing the failure.

Either the device_type should be reverted back to what it was (i.e. not a vendor id) or a new field should be added to the device to indicate the type of that device and then update the code accordingly.

Once this change is done, the test_persistence integration test MUST be run to ensure the restart logic still work.