JSONSchema: Adapt MAC address format to unicast

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.

[0] https://lists.linuxcontainers.org/pipermail/lxc-users/2010-August/000783.html
[1] https://en.wikipedia.org/wiki/MAC_address

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>