There was an issue in pseries llan driver, that resulted in the first 2 bytes of the MAC address getting stripped, and the last 2 being always 0.
In most cases the network interface still worked, despite the MAC being different of what was specified to QEMU, but when some other host or DHCP server expected a specific MAC, this would fail.
This change fixes this by shifting right by 2 the local-mac-address read from device tree, if its length is 6 instead of 8, as observed in QEMU DT, that always presents a 6 bytes value for this property.