MAC-addresses having the LSB of the first octet set, are considered
multicast-addresses (see [0,1]).
the 'mac-addr' format got changed to only permit unicast addresses, which should
work for its current use-case (WOL for nodes).
additionally a default option was registered via register_standard_option to be
used in both PVE::LXC::Config and PVE::QemuServer.