Improve device serial number consistency

[nefarius]

Currently the hex-encoded MAC address of the remote device is spliced into the Device ID string to enforce uniqueness and consistency. This behaviour changed slightly with the introduction of the DMF_Pdo module. Suggested new design:

• Check the drivers Parameters registry key if a sub-key with the hex-encoded device MAC address exists.
  • If it exists, attempt to read the DWORD value SerialNo and assign it to the PDO
  • If it doesn't exist, find the next free serial, create the key and store the serial in the DWORD value SerialNo
• Once the device is reconnected, it is ensured to end up in the same slot again by using the registry-cached information.
• Save and restore the slots value as a REG_BINARY in the registry as well to avoid duplicate assignments